M'Kay. There are a couple of things missing here. Did you use a tape measure and get the exact points every time? I'd recommend pushing a pin as far back in the tail as possible, tie some string around it and measure. That way your sure one point is always at the same location. Since you've already glued the dowels in place, use a Sharpie to mark the string and move to the opposite wing. If your concerned, you can always slightly widen the holes in the WING and slide it left to right to get the measurement a little tighter.

Okay, to much information. As for the 3/8" difference, it won't make much (if any) difference. I built my 4*60 and had 1/4" of slop, with no issues. Unless your a world class 3-D'er, that distance won't effect the plane. My thoughts. Any others out there?