Here is a bit of recycling in Austerity Britain! A surplus subchassis, a bearing with a cracked jewel and a few wood scraps - I can bolt a blank arm board in place, mark the spindle to pivot length off on the ruler, put a bit of masking tape in place and mark an arc of suitable hole centres.
I have used it for checking existing arm boards and making new ones.
P.S. Please don't ask what the <-18mm is about; I can't remember! Might be to do with the location of the spindle hole in relation to the end of the wood.