Please I need math help with churn dash

I am working on a quilt and the last two blocks are both churn dash. One squares up at 5 inches (the center square is cut at 2 1/2 inches with 1 1/4 in rectangles. The other squares up at 12 1/2 inches and the center block for it is cut at 5 inches with 1 1/4 in rectangles). I cannot figure out what size square to start with to make the HSTs in the corner for either one. I am ready to throw this quilt away!

If anyone can understand what I am trying to say - could you help?

I think the measurements in the instructions are wrong but it very likely could just be me. All I know is I need help!!! LOL:confused:

Thanks for any reply.