It's 5nm, and the easiest way to calculate the Mach number to use it to ask the one in front what they're doing and tell them to fly that or greater and the one behind does that Mach number or less. That should hold the gap at what we want.
Experience will tell me before I even ask if it's q question worth asking. For example, I won't try and run a 737-300 in front of a 737-800/A319 unless I absolutely have to in the cruise as most 300 series don't want to go above .75 and the 800 will want to be about .78. In the descent I'd happily do it.
In the case where the back one doesn't want to go that slow or the front one doesn't want to that fast then we negotiate or different levels are used or a direct routing for one if they're splitting up later on or as a last resort radar headings.