Hi Caroline!

This quilt block is a 4 patch made up of 4 identical units. So (with the base of the triangle on the bottom) the Kona black will always be on the left side of the triangle. In your photo, the yellow unit looks correct (but the light purple units appear to be flipped).

Your units are big enough (I can tell by the measurements on your mat). You can leave them oversized at this point and trim down when you complete the square for the 4 patch. That's what I did (as I found it easier). But if you want to trim them down now (and to answer your question), the two short sides of your triangle need to be at least 6 7/8 which means the long side (or base) of your triangle will be just shy of 9 3/4. Whether you trim down now or not, the key is to have the seam (where the black and yellow meet) be at the mid point of the base of the color 2 triangle. This can be done by folding the triangle in half to find the midpoint. Quilter's Cache says to cut color 2 at 6 7/8, but I probably cut at 7 1/4 or larger to give myself plenty of wiggle room. I then folded the resulting triangles in half to find the midpoint and pinned to the midpoint of my half triangles (and didn't worry about my edges not lining up as those would be trimmed).

Each 4 patch needs to be 6 1/2 so when sewn together will result in a 12 1/2 block.

I hope this answers your question! If not, please, please, please let me know and I will try again!

Jennifer