Generally the ( formula) for sashing is a number that goes into the measurement of the block, so 7” would be ( balanced with 3 1/2” sashing, or, 6 1/2” with 3 1/4” . Or smaller. If you want 5” between the blocks in order to make it the size you want you might consider multiple strips, like 3 1/2” with a 1 1/2” strip on both sides of it.