Shirt collar fit is a real pain because so many people have necks that change size quite quickly with the addition or loss of a few pounds. personally I hate when a collar is too loose because you really do look like you have been sick and are just now recovering etc. With a too-large neck comes a too-large body often times as well.
another problem is that some people's neck to chest ratio isn't the industry standard so shirts cut for someone who has a 42 chest might need a 15 neck but can only find them in 16 so great - the body fits, now the neck is loose.
In the case that the collar is too tight your neck can spill over it making you look bloated but a quick fix for this is a collar expander which looks like a button on one end and a springy loop on the other, the loop goes over the existing button and you use the attached button to close the shirt collar and since its elastic you wont choke yourself to death. This only works with a tie however.
I dont use the "two-finger" test because I know what I am looking for - a collar that doesn't bind as I turn my head, that I can get a finger or two into like I was going to tug on my collar and one that stays up high in the front of my neck. I eye-ball it at this point and I know when its what I want.
If you are wearing a jacket over a shirt as long as the collar and sleeve length are good if the body of the shirt is a bit too full the jacket can conceal some of it but the neck is never concealed. Cuffs spilling onto the back of your hand is as bad as a bad neck fit and the two together scream "shirt too big!".
Short of custom shirts or MTM shirt programs you are reduced to hunting down shirts by brand that offer the neck size, sleeve length and body fit combo that works best, it however almost always requires some compromise.
The best you can do is try before you buy, maintain your weight plus or minus 5 pounds and take good care of your best fitting shirts.
Also note that a shirt that is almost right, maybe just a pinch on the big side, may fit just right after a few washings but dont expect too dramatic of a difference. It would be much more noticeable in sleeve length than in neck diameter. Conversely if the neck just barely fits brand new it could become even tighter after a few washings.
You want a "science of fit" answer but I think the only real answer is an "art of fit" answer, sorry...
