Take a piece of string and using a T-pin attach it to the middle of the engine firewall. Stretch the string until the slack is removed, place the loose end at one of the stab. edges, move it to the other side, adjust stab as necessary. When both sides measure the same it's square to the fuse. If your off a hair, it's not that big of a deal. The stab and vertical fin can be trimmed out in flight if they are off a tad.