So I'm loving the tube strip method, but have not figured out how I can end up with a specific size block.

If I'm using 2 jelly roll strips (2 1/2" ) my finished squares are 6".

I want to make finished blocks 3 1/2" blocks so that I can make 12" blocks (12 1/2 unfinished).

How do I figure out what size to cut the two strips? There must be a formula ...

Yes, my terrible math skills have caught up with me :(

Thanks!