Jack,
The best thing about those plastic stocks is that you can easily adjust them with a propane torch and they very rarely ever crack.
To answer your question, if I were to measure dac or dah on a gun with the barrel below the receiver I would lay the gun on it's side and put two spacer blocks of known thickness on either end of the barrel. Then lay your straight edge along the blocks and take your measurements. Then simply subtract the thickness of the spacers and you have your actual measurements. Hope that makes some sense.
Threaded Mode