I would like to do a lap size D9P quilt with 5" charm squares, and I was wondering how many squares I would need. I did a search on the board and there was a question a while back about the finished size of a block with this size square. It was 131/2". I did some figuring and would appreciate it being checked. If I do a 4x5 layout, it will be 54" (13.5x4) by 67.5(13.5x5). 4x5=20x5=180 squares. If I did a 3x4 layout, it would be 40.5x54. 3x4=12x9=108 squares. These sizes are without borders. Sometimes I think I have some things all figured out and realize later on I made a goof and don't have it right! So did I fiqure this correctly?

Thanks