I'm seeking some advice on how much of the roots I can cut off when I do it
Up to one third - but you may want to take a more cautious approach. Seems the tree is in a high pot, you should not try to get it in a shallow bonsai pot right away - but reduce the roots over the course of a few years. Depends also on what you find, how big the roots are, how deep they go, if they are long and winding...