This is a great method to make quick HST but please remember that all of your edges will be on the bias, so use starch or be very careful not to stretch when piecing your blocks.
Another thing you can do is cut your original large squares on the bias then the cut edges of your HST units will be on the straight grain.
Using the formula in the link above the squares should be 10.15625, so I would cut them at 10.25 and trim each HST unit to 6.50. When you sew the four HST together you will get a 12.5 inch pinwheel block that will finish at 12 inches. Hope this helps