My pattern says cut (68) 3" squares, then cut diagonally for pinwheel parts (times 2 fabrics)

I will end up with a total of 17 pinwheel blocks which are ultimately 4" square.

If I were to use Jennie Doan's easy pinwheel method of sewing all the way around two pieces, then cutting diagonal both ways, do I need to start with 4.25" squares?

I've drawn this out on a piece of paper, and measured, and I get the whole pythagorean theorem, but just need to confirm. And the second part of the question is: if itisthe 4.25, do I need to be concerned about the decimals? (square root of 18 is 4.24264068712).

Should I bag it and just cut the flippin triangles?