Years ago at design school, for one lesson a pair of professional furniture designers showed us the simple buck they made to work out seat and seatback angles: It was two large metal wire grids, facing each other like the wings of a TIE fighter. They would slide thick dowels through the grid from one "wing" to the other, building up the surface of the seat and seatback, then they'd sit on the dowels to assess the comfort of the angles. It was quick, dirty and clever.
I want to say they were using 1.25"-diameter dowels, which gives you an idea of the grid spacing. It's only going to get you so close, and for further tweaking a more refined buck would be needed. For greater, literally millimeter precision, you could build something like furniture designer Oliver Trepper demonstrates on his Instagram:
I'd love to see more chair buck contraptions, but unsurprisingly they're not easy to find. If you've got an example of yours and you're not proprietary about it, please do drop us a line.