When building a foam wing, do you measure the length of the wing before the dihedral is cut?

On the Terminator I am building I have cut my foam blocks to 20.15 inches, but just now realized that once I cut in my dihedral that the top surface of the wing will be less than the bottom surface. Even in the original plans it says the outboard tips are 20" - is there a standard way of measuring the length?

I hope the answer is that it is ok to measure along the bottom of the wing as the cores are already cut and awaiting balsa sheeting!

I really don't think a difference like that is going to be a critical issue... Just go with what you have, and if you're really that worried about it, next time account for the extra .15 inches and add a little to the cores