A template was made of plywood to trace the voussoir shapes we would need on the chunks of red sandstone
We determined that the voussoirs would be ay least 8 inches at the base (intrados) to fit the dimensions of the arch circumference we had to span and still make the best use of the sizes of sandstone blocks we had available. The sides of the template were then determined by the radiating lines coming from the bottom centre of the form. (the point where the trammel had been fixed to the base)
While it went pretty smoothly tracing and then cutting the chunkier stones into voussoir shapes, there was some difficulty getting the correct angle on any stones that were slightly smaller than our template. Any discrepancies in these angles had to be 'fixed' ( chiseled down with a point) after we tried fitting the arch stones together in situ
The two huge keystones were then lowered into place exactly top centre of the form.