Distance required to lose specific altitude:
(Altitude to lose x GS) / (Rate of Descent x 60)
I use this one if I want to be at a certain flight level and know my desired descent rate. So, if I’m at FL350 and want to hit 11000 by 50 nm out, and want to descend at 1500 FPM, I multiply 24000 by my groundspeed, then divide that by the product of 1500 and 60. The parentheses are very important.
Rate of descent required with known distance:
[ (Altitude to lose x GS) / Distance to target ] / 60
I use this as I proceed from my start point established above. Obviously, your speed decreases as you get lower and lower, so you won’t need to descend as fast, so I use the above to re-evaluate my ROD.
Say by FL220 my groundspeed has decreased significantly, I then use the distance remaining to the target waypoint (where I want to establish a cruise at 11000 for a short time or something) and the second equation to lower my ROD. I continue to evaluate this throughout my descent.
There is no simple calculation that will give you an exact number which doesn’t require revision during descent. It’s impossible to account for the consistent loss in speed as you descend.
There are plenty of threads on here about descent. If I recall, you’ve asked about it two or three times before, so I’m sure you’ve been pointed that way.
The thing is, it’s not that difficult. Just some simple arithmetic and algebra, and I’ve done the algebra for you already.
I don’t do rules-of-thumb, though there are a million out there. These two equations and a few seconds of conceptual thought take care of any and all of the mathematics of descent for me.