Your block FINISHES at 10.5 inches. That is the size each block will be when they are sewn into the quilt? Just making sure here. That means that each of your four HST's for the pinwheel will FINISH at 5.25 inches. So add 7/8 inch to the finished size 5 1/4 plus 7/8 which would make 6 1/8 (or better, add 1 full inch and trim down to the exact size you need.
But if your block is now 10.5 inches, that means that the FINISHED size will be 10 inches and for the pinwheels, you need 4 squares whose FINISHED size would be 5 inches. In that case, you are correct, you would cut them at 5 7/8 inches. Again, I would round up to 6 inches and trim.