Actually, giving a person the recommendation to use the 3 finger method is quite silly. I mean, we all have different sized appendages, and 3 fingers for one may be 4 fingers for another.
Also, when we have a chain tensioned correctly, there is a very fine line..and only about a 1/4 turn of the tension adjuster, to take the chain into way too tight.
Providing pictures of your bike and claiming a person should tension it the same is crazy. The tensioning procedure is far to easily hosed up to simply look at someones picture and be able to make anything at all from it.
I have studied the proper adjustment of chains and sprockets for years. I find that one of the biggest problems folks have is misinterpreting the manuals suggestions.
For an example, the latest Yamaha 450 manual gives a check range of 1.9-2.6 inches from the back of the top chain slider to the bottom of the chain. After doing a check, I find that this measurement range is about right on. But, do you check this while the bike is on the ground, or while the bike is on the stand? And there lies one of the discrepancies I mentioned....
But where do these moto engineers get the proper recommendation for chain tension in the first place? They use standard engineering practice for chain tensioning...and they pass that textbook information along to the owner, taking into account the length of chain on that particular model bike. The engineering practice they use is the very same for each and every bike, but what changes is the numbers due to how long the chain is...or closer yet, to how long the distance between shafts is...which is what actually determines how taught a chain should be.
If you were to use the engineering standard to tension your chain properly, you could then go back and check the measurements that the manual gave you. And you would find that they are indeed very close to what you found. But only if you interpreted what the manual was trying to convey to you properly.
I see so many folks who will swear that stock OEM chains are nothing but cheap junk and should be shucked immediately. But that is not really fact. Yes, the OEM chain may not be the top shelf quality chain that your moto supply may want to sell you, but it is almost assuredly of a good quality that will last a rider for many rides before it is trash...IF the rider has it adjusted and maintained it properly. An improperly adjusted chain can only live so long..and even the high-dollar choices will be trash fast if they aren't properly mounted.
I'd have to say that the biggest mistake I have seen over the years is folks having their chains too tight. When a chain that is too tight lands a jump and compresses the shock, the overly taught chain eats away at the sprocket teeth, the chain itself, wheel bearings, CS seals, and on and on.... SO many times a rider will then swear his stuff is crap, and start looking for harder and more robust equipment that can handle their improper adjustments better.
I contend that if you properly adjust your equipment from the get-go, you will find that even the OEM equipment will provide many hours of riding. Your high dollar replacement equipment will also last longer.
You first need to put the bike on the stand and remove your shock. Before you do anything, simply take the swing arm through it's motion of travel from top to bottom. This is the point where many are convinced something is amiss right away, as they often find that there is a point in that travel that the chain gets completely tight. Bowstring tight in many instances. This often will open a persons eyes who has thought they were tensioning their chain properly, but were in reality over tightening it.
The tight spot will be when the CS, swing arm, and rear shaft are all in perfect alignment. When you have the bike in that position, you want to use a cargo strap around the seat and the rear wheel to hold the swing arm in that tightest position.
Once you have the swing arm in the tightest position, you can then adjust the chain tension. The engineering standard for chain tension is to have between 1%-3% of the distance between the front and rear shafts in total up-and-down chain movement when the chain is at it's tightest point.
For instance, if we have a bike that is 24" between the CS and rear shaft, the correct tension for the chain will be between .24" and .72" of total up-and-down free play of the chain when at it's tightest position. Knowing this measurement, you can initially adjust the chain to the .24" mark, and retention once you get to the .72" mark. If this is done, the chain will always be within the recommended tension rage, according to engineering standards. Some may find they feel more comfortable staying within the 2%-3% range.
Let's assume you want to start out with the 2% mark and retention when you reach 3%. At 2% of total up-and-down free play would be .48"
Note that the tension on the illustration shows the chain having .24" of free play when it is pushed down. And as such, it will also be able to move when pulled up by the same .24" This means that you have a total of .48" in total up-and-down free play in the chain. You are tensioned at the 2% mark.
Once you have the tension proper, you want to make certain you still have proper alignment if the sprockets. When everything is nice and aligned, and you are satisfied that you have the proper tension on the chain, you want to make certain everything is buttoned up tight, and you can replace the shock. It is best to recheck everyting before you replace the shock, as things can change a bit on you once you have the adjusters and axle bolt tightened. Take the time to check and recheck until you have it spot on when tight.
Once the bike is adjusted properly, and back in running condition...THEN you can check to see what that properly tensioned adjustment gives you when the bike is on the ground...or on the stand for that matter. What you want to do at this point is have some sort of reference so you can check the tension without going back through the shock removal procedure again.
If you find that the properly tensioned adjustment gives you three fingers under the chain, behind the slider, when the bike is on the ground...then fine. Use that to determine if you are properly tensioned. But don't tell anyone else that is where they should have their adjustment, it simply may not be correct.
To take this further, which is what I do whenever I get a new bike, is to first tension the chain at the upper limit of tension or the 3% mark. I then button everything up and check to see what measurement that gives me when the bike is on the stand. I can from then on see with an easy check when my chain reaches a point that it needs to be re-tensioned. I then go through the whole thing again adjusting the tension to the 1-2% mark, and recheck to see what measurement that gives me when the bike is on the stand. I now KNOW what measurement I should have for properly tensioned chain and chain that needs to be re-tensioned, when the bike is on the stand. I never have to go through the painstaking procedure of removing the shock to properly adjust for tension again.
Some will actually cut a GO/NO GO block of wood or plastic to use as a gauge. And a proper gage block will have the distance of a properly adjusted chain on one side, and a larger measurement on the other side that will tell when the chain has gone further than the 3%.
Many folks have gone for years improperly adjusting the tension on their chains. Many simply accept that their equipment wears out fast...and some find excuses for it like, the chain is junk, or their beastly bike simply is too much for the chain and sprockets to handle...but neither is the usually the case. The truth is that their tensioning procedure is placing undue stress on their equipment, and if they would take the time to do things a bit differently, they may well find that stuff starts lasting a lot longer.