Sometimes accuracy is important, and this simple grid, attached to a batt (compatible with the nesting-batt-system I previously posted about) can help.

These are designed to be cut from 6mm ply. The board with the grid slots into a batt, and both pieces are glued in place. (again, I used Araldite, for speed and strength.)

Set the pointer of your Throwing Gauge to the required size, and then swap out the batt and begin!