This rule of thumb might help...
If you're flying a DME ARC:
Every 60 degrees is about the same as the inbound leg (or ARC DME) (less than half a mile difference for a 10DME Arc)
OR even easier:
For 10 DME Arc, every 6 degrees is a mile
For 12 DME Arc, every 5 degrees is a mile
For 15 DME Arc, every 4 degrees is a mile.
Simple math will show that these figures are close enough to be exact for thier intended purpose.
I am sure you are not too lazy to work out the pattern in the above....
