A digital protractor from your local hardware is the cheapest option. They have a magnetic base, so you could glue a metal strip to the top of your quill base for it to sit on.
They work well with a dial gauge as you can rest the stone on the lap. Adjust the mast height till you have the angle you want, take the protractor off, note the dial gauge setting and start cutting. Using the dial gauge as a stop you will be able to maintain that angle with that row of facets.
The protractors from your hardware are accurate to about 0.2deg, the one in the picture, which I got from eBay, is more expensive and claim an accuracy of 0.05deg.