Derek Cohen (in Perth, Australia)
The answer is so obvious I wonder if we are talking about the same need.

Pin board in vice; tail board held in registration over the pin board to enable transfer of tail profile to pin board.

I have seen many do this operation and all simply push down on the tail board with one hand and mark with the other. I have never had any problem holding these boards in registration while marking.

Try doing that with slim ("London") dovetails. I can't. Bet you can't.

Regards from Perth

Derek