I am thinking the rule of thumb is for any block size is " finished block size + 1 1/2( this will allow for the cutting of the block into 3rds) : then size of block divided by 2 ( for HST) +1/2"= size of blocks to be cut for HST.
EG. If I want a 12" finished block - 12" + 1 1/2"= 13 1/2"( that is the size of the unfinished block to be made into pinwheels) To determine block size for HST 13 1/2" divided by 2= 6 3/4 then add 1/2 for sewing on diaginal. so block to be cut into HST is 7 1/4. I will try this soon to see if it works.
I think this way sound easier that making 2 blocks and then cutting and sewing them together. Also the 1st pic Roolen shows is with origianl block in pinwheel.