On HST's, regardless of what size, I cut the 2 squares an inch larger than the HST(unfinished) is supposed to be. In your case if the HST's are supposed to be 3 1/2", I would cut my squares 4 1/2", a lot of directions only allow 7/8", and sometimes that might work, but I'll just be a little more "wasteful" and allow an entire inch.