Get the airplane on a flat surface and block up the tail until the firewall is 90 degrees to the surface. Then you can measure from the surface up to the LE and TE of the wings and stabs. Don't be afraid of needing to make changes. I have elongated holes for anti rotation pins many times. My favorite method for this is to file the hole slightly larger and the direction it needs to go. Then I will epoxy a fiberglass tube sleeve in place. This works out great as not only do you get the change but the anti rotation pin no longer runs on wood.