I'll tell you what has worked best for me: I was making HST out of 5" charm pack and I was going to make a lot of them. I knew that my finished size was going to be 4". I went to a glass shop and had them cut me a 4" square out of plexiglass. When I got it home, I took a Sharpie and drew a line diagonally corner to corner on the back. I then put on InvisiGrip to keep it from slipping and to keep from wearing the Sharpie line off. I placed the plexiglass template on each HST, lining up the drawn line to the seam line. I then trimmed the squares; sometimes I had to trim a little off each side, sometimes a little off one side only. When I put the squares together to make my blocks, the centers came out perfect. I have since gone back to the glass shop and had several different sizes made to trim up different size HST. My shop charged a couple of bucks for each. Just make sure you take a ruler or tape measure to measure the plexiglass templates before you leave the shop so you know they are the right size when you get home.