This job can be done with a jury rigged marking gauge, a sharp chisel, and a file or two.It takes time and some skill, but it's the way it was always done before people had plug-in tools. Normally in that case you'd mark the purfling line on the top first, take that down, and then mark the binding line, so at this point you're bass ackwards. A shallow gauge that runs on the binding rabbet would work. I'd make one by sticking a guide pin into a stick sideways, to guide on the rabbet, with a sharp two sided cutter to mark the top at the right distance, if I had to do that.
