In the higher levels on descent then feet per mile to lose X Mach number (which is close enough to miles per minute) should give ROD needed in feet per minute.
E.G. You need to lose 6000' in 20nms, 300 feet per nm, at Mach 0.8 you will need to decend at 2400 feet per minute.
On final approach 5 times your groundspeed.