Beyond RoSpeed <+10>, it is entirely luck. The basic speed ring is 2+M8, which has a minimum speed of 3 and maximum of 10, with higher numbers much more likely deep in the dungeon. Once you do this, there is a series of coin flips, with each successive "heads" adding one to the speed. So +20 is around 1 in 1024 once you are down that deep.
