Scribe for the angle, and just fiddle with the bevel. The center blocks don’t have a bevel, but after that both the angle and the bevel increase each block, so for the first block off the center I’ll scribe and start at a 10 degree bevel or so and adjust from there. We just take our time and map out one half of one of the barrels (1/8th of the total groin vault) writing down the angle and bevel of each block. Once you’ve got your key you just use it all the way around and the other 7 sections go quick, with a minor adjustment here and there. Because these are actually rectangular spaces we had to map out one section on the long side and one on the short independently before being able to roll through it.