I don't know exactly, but I'm about to start one that will finish 65 inches and the pattern calls for cutting the squares at 6 3/8. I also have a pattern for one that finishes 70 inches and you cut the squares at 6 7/8. So, maybe if you took a half inch off for each 5 inches smaller you want the quilt, you will have it. Let's see: So the way I figure it, you need to cut them at 4 3/8. Good luck!
Doug