Here is how I use the Tri-Rec rulers. I start by cutting a strip of the center fabric (red) the UF measure of the block. Then line up the top (small) flat edge of the center triangle ruler with that and cut.
For the sides (nuetrals) I cut the width 1/4" wider than the UF measure. Also line up the Top (Flat) edge of the side triangle ruler. I don't cut any of the little bits on the corners as I won't be using them to line anything up.
When I sew them, I sort of split the difference, with about 1/8" over on both top on bottom. Doing this gives a bit more on the sides and top for trimming.
When I sew the second side, I can line the top of the second side with the top of the first and go with that.
When trimming, it's important to get the point of the inner triangle centered and at the 1/4" mark. (the ruler was placed randomly, the 5" mark has no meaning here)
The two outer corners should line up like that next picture. If you end up with the unit being too short, you are sewing your seams a bit too wide. This is one block that gets picky about keeping the seams right. If you sew to narrow, you'll not have enough fabric at the top for the 1/4" seam allowance above the point (my biggest problem if I don't oversize the sides).
The last picture shows about how much is trimmed off the block.