The one I have memorised is from RANT:
NMs before BoD = (Height in 000s * (GS/60))/RoD in 000s
Looks messy, but you will know the GS/60 before you fly.
If you want to know what RoD is required to meet ATC BoD request then just transpose to
RoD in 000s = (Height in 000s * (GS/min)) / NMs before BoD.
This KTs/min is used in many mental math calcs so it is worth memorising cardinal points like 90kts, 120, 180, 240, etc.
If you need to mentally divide by say 2.5, then I find it easiler to double both operands first to remove the decimal.
Easy peesy
