That is really impossible to day as each gun will be different, even from the same maker. Some folk make a dummy round (no primer of powder) and load the bullet deliberately long, mark the bullet with magic marker or candle smoke and chamber it in the desired rifle. If the rifling marks the bullet, extract, set it back slightly in the seating die and repeat til the rifling no longer marks the blackened bullet. Set it about 1/4 to 1/2 turn on the seater stem and record the OAL for future reference.
It is a good idea to start at MAX OAL and check to make sure your magazine will take that length. Adjust down from there.
Just get the stoney point OAL gauge and compairitor
They are not that costly and work great. I love mine and sometimes get it out just to lay with it.
If you want to use the bullet in the case method you need to inside neck turn the case to allow for less neck tension on the bullet. This is going to be hard because you have to have enough tension to hold the bullet as you eject it but you have to have just enough to allow the bullet to move in the neck.
I'll also reccommend the compartorer. But if you only have one rifle and you don't want to spend the cash of a comparator, you can probably make a workable substitute by drilling a hole thru the primer pocket of a fired case and inserting a rod (or whatever) into it and a bullet into the case mouth. Remove the bolt of your rifle chamber the case and push the bullet until you feel the bullet touch the rifling. Mark the rod (or whatever) even with the head of the case and do the math and measure properly, and you can figure the COAL.
I bought a comparator, but you have to have a separate case for each caliber. I drilled and threaded extra cases and use the rule in the comparator. But I don't see why you couldn't make a reasonable one.
With the comparator or with pushing the bullet with the bolt, as the OR desicribed, the rifling will grip the bullet and displace it, and (especially pushing the bullet with the bolt) you can never be quite sure you're getting a good and accurate COAL measure.