"You can take that and add it to the original sine wave via voltage
dividers (a decade resistance box would be useful), and calibrate the
meter down as low as you like. 90% sine + 10% square = 4.84% THD, etc. "

Very interesting. Once I got the thing to the point where I think it works I will have to look into that. An actual ay to calibrate it, though not really accurate it does prove it works. Maybe with a little bit of trouble it can be confirmeed down to a certain resolution.

The Wavetek has a fixed square wave (in fact all of them) and a variable output that is selectable. Couple of resistors should do it.

So 90/10 should read 48.4 %. That means 99/1 should read 4.84 % ?

Question - am I going by peak value or RMS/average or what here ?