Just as a disclaimer getting legal advise from any internet forum can be risky, the information may be incorrect. Just wanted to throw that out there.
I have done a number if F1's.
First my list of items attached to my trust is transmitted in a schedule E, which is updated every time a stamp is added to my trust.
As for measurements, I use the method hdfireman mentioned above. Although all stocks have different lengths, so to muzzle devises. So I take those two times into account and if possible install them, then minus the number of inches for the barrel.
As for running different barrel lengths on a single registered lower I have heard a bunch of different versions. One being you can only run what is on your F1, also what hdfireman mentioned, and the you can always run longer but never shorter than what is on your F1.
Personally I do not care to mess with this and just stick with a single length, figured that is the safest if it ever becomes a question mark. Some of those other versions may be right, but I don't care to push it.
FYI currently F1's are taking about 4 months for approval. The days of 2-3 weeks have not reappeared.