This is one of those situations where the big Hobbico "Builders cutting mat" with the 1" square grid lines can be really handy.

First off, I would suggest the possibility that the firewall is NOT "square" or perpendicular to the centerline of the fuselage, but has right hand thrust built in. Therefore, the marks being to the left of center will allow the engine, out at the crankshaft end, be centered on the centerline of the fuselage. This is where the Hobbico mat with the grid lines is easily used to line up the aft end of the fuselage on the grid as well as say the wing saddle areas on each side of the fuselage. It would be readily apparent, probably wouldn't even need a drafting triangle to determine that the firewall is indeed built on a skew to allow for the proper right thrust.

Of course you could also stand the fuselage up on the firewall (assuming the triangle stock is the same on both sides) on a flat surface and use a builders square to see if both sides of the fuselage are square to the surface, or if one is more than 90 degrees and the other is less.

Rube