i would eliminate those four orange triangles immediately surrounding the focus fabric. i would make the focus fabric the size of that complete space, and then go directly into the little 'basket' shapes. your central fabric would have to be on point to end up with the same shape as the one that includes the orange triangles.
if you want to keep orange, or whatever color you're using, in the mix, then cut your focal fabric on point leaving enough space to frame it with a tiny border of
that color. go from there. you'll end up with an on point shape, as shown, with the same color combo rearranged to suit your desired size.
that way you don't have to do the math.
edit: i don't know if i explained that right. picture the orange center as a solid piece. picture your focus fabric the size you want it, but on point. now think of the focus fabric on top of the orange, centered. this way, you're only making 2 changes, both minor, imo. one is twirling the center. the other is changing the size slightly. you still end up with the same size middle with an orange surround. the september block also has an on point center. are you doing all the blocks or is this your chosen one for this challenge?