I purchased 2 lowers and uppers. They are aero brand.  First one is going to be a .300 blackout with 8.3" ballistic advantage barrel and the second will be a 5.56 10.5" barrel.  Both firearms will have the magpul ubr gen 2 stock on them (when they are available) with the max 12.5" length that the specs state on magpuls website.  I know that you have to measure it fully extended but do I measure from the end of the butt stock extended to the end of the threads on my upper receiver that the barrel gets attached to and add the length of the barrel such as the 8.3 and 10.5"?  Please help I just want to be certain I have the correct length before I submit my form ones. Thanks  

The method you describe will be fine.  Technically you will be a little over reality just a tad, but the ATF doesn't need exact measurements, you just have to get close.

thanks

If you plan on using a conventional stock (non PDW)

Barrel length + 19" = OAL