orientation of the cut is a bugger. Expect a very uncomfortable amount of waste.
Generally, you will find one cut gives you birds eyes and the 90 cut to that gives you rays, like sun rays. For other burls, you will see swirls and straight grain. It depends on the wood. You have to decide which cut you like best and want to show on your handle. You then have to orient your cuts to maximize what side you want to show on your handle. I normally make a big cut down middle if the burl is large. I'll cut it 1/3 if the burl is smaller. This is to see what is going to show (eyes or rays) and to give me a flat side to cut from on my band saw.