You're going to need some strange sizes to get a 10.5 inch square. The closest I could come up with are strips cut at
3 7/8 inches which totals 11 5/8 total but you lose one inch when you sew the strips together giving you 10 5/8 as close to 10.5 as I can figure. Maybe just make your seams a thread wider than 1/4 inch. Or just sew some sample strips together and see how they measure. Hope that helps. This should work if your center square is 10.5 unfinished.