Here's an equation I used for d9pTo calculate the size of the finished block you need to multiply the size of your original squares by three,
divide by two, then take off one. This will be the finished size of the block.
For example if you begin with 5-inch squares the calculation would be as follows.
5 x 3 = 15, ÷ 2 = 7½ - 1 = 6½ inches.
6-inch squares would yield 8-inch blocks
7-inch squares would yield 9½- inch blocks

Usually when people are talking about d9p blocks, they mean the size block you get when you slash the 9p block into 4 pieces and then reassemble it. Your formula is for the blocks after the 9p is slashed into 4 pieces, but before it is reassembled. To get the size after reassembly you would have to multiple by 2.

Lordy! Too much thought for a quilt block. Use 5 inch squares and make the 13" finished block. The quilt will have extra toe room.

You are right - I forgot I multiple it by 2!