I tried 70 ipm previously (as per Phil Johnson’s recommendation) and the results were quite a bit worse than the 35ipm that I used in my latest carve. Depth I now have set to 0.04 and that seems to be close to correct. The burning on the border was my fault because I had the depth set to 1/8" for a single pass and that was just too deep for my X-carve to handle in a single pass. And my bits were brand new when I started this project.
As to my belts and v-wheels I have done my best to tune them in just right and I examine them before each carve to make sure they are correct. As recommended elsewhere in the forums I bought a fishing gauge so that I could use it to precisely set the tension on the belts.
What else I might do escapes me.