I use a luggage scale I picked up at the hardware store. It records a stop at any draw point. Here are pics that explain it better than me. The one pic is sideways.
The scale hanging by it's handle from two dowels in my back porch cover frameing.
Sideways, but how it hangs from dowels. The slat of wood is marked with inches starting at the hook. the bow string goes on the hook and the bow is drawn down along the measure slat.
As packaged when purchased.
The memory dial makes it easy to build an FD curve by recording a series of weights and then plotting them. By drawing the handle down the measure slat you can pick any point between the back or face side of the arrow window to be your own "tru draw" point.