Just popped in to see what you are all up to. Someone, sorry, 2 or 3 pages back, said that she would have liked to make wider sashings, but then the triangles wouldn't fit.
I used 3/4in sashing, instead of 1/2in, and basically made my in between triangles a little larger to compensate.
You don't even have to do any maths, just cut out some paper squares the same size as finished blocks, plus some strips the same size as your sashings, plus a few triangles. Stick them down on a table top with some sticky tape, and play with the size of the in between triangles! Of course, if you are computer savvy, the software should be able to do it.