Actually, you needed larger triangles.
Instead of doing a lot of calculations, I simply cut the triangle units a couple of inches larger than necessary. I find the center of the triangle side (usually fold in half and iron in the halfway mark) and do the same on the center square (find the center of the side and iron in the fold). I match the center points and sew the triangle on, iron, and then use a large ruler to cut the triangle down to the correct size. This works for side triangles when assembling a large quilt of on-point squares also.
Edit: Found this website for the math:
http://quiltville.com/onpointmath.shtml
The math you need is the corner triangle directions (first ones on the page). Instead of using that exact math, I just make the squares bigger. For example, instead of adding on 7/8ths of an inch I would add on at least an inch, and probably more like 2 inches (just in case!).