Hi Terry,
You may find the answer in doing a couple of test cuts, one on the table saw and one on the sliding mitre. I think if I were going to do this, I would opt for a table saw and use a "taper jig" (sometimes called a taper leg jig. You can find a plan for this if you click on our "Free Woodworking Plans" link .... scroll down to the bottom to "X-Tremem Woodworking Files" That will take you to a free plan for an Adjustable Taper jig, and there are of course many others on the web.
You could set the jig at the 20 degrees you need and leave it there, that way you will always get consistent results.

Just my 2 bits worth
PS ... would love to see the Lute either during building progress, but certainly after. I love these old world instruments.
Colin