Cougar.
Are you asking mach no. technique? That is an actual standard in procedural (non-radar) separation. For a difference of .05M, you need 6 minutes (faster ahead, obviously) at the same level.
If you are in a radar environment (to be able to vector), you wouldn't be using MNT. But you might use mach no. for flow/sequencing purposes.
About the only rule of thumb that I can think of in the app phase; if you have 2 perfectly spaced and you want to keep it that way, reduce the second one 10kts IAS slower than the front one (the higher a/c will ground out slightly faster). It gets complicated between different types (A330/340 get down low earlier).