Another possibility. Takes a bit more calculation, but instead of pitch, you can also use rate of descent and 1-in-60-rule as a good enough approximation.
What’s the 1-in-60-rule?
Your ground speed (knots) is the distance (NM) you travel in 1 hour. Divide that by 60, and you get your distance travelled in minutes.
E.g. 360 knots GS = 360 NM / hour / 60 = 6NM / min
In the International Standard Atmosphere (ISA), Mach number x 10 = NM / min. So Mach 0.60 = 6NM / min.
So in ISA, when you fly Mach 0.60, your ground speed is 360 knots, or 6NM / min. If you fly 420 knots, that’s Mach 0.70 or 7NM / min; 480 knots = Mach 0.80 = 8 NM / min and so on.
Now that we know that:
Imagine you’re at 22,000ft, Mach 0.80 (hence assume ground speed = 480kts), and you have 64NM to descend to 4,000ft.
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64NM / 480 knots = 64NM / 8NM/min = 8 minutes
22,000ft - 4,000ft = 18,000ft
18,000ft / 8 min = 2,250 ft/min
Similarly, imagine the same scenario, except you want to calculate the distance you need for a given rate of descent.
22,000ft - 4,000ft = 18,000ft
18,000ft / 4000ft / min = 4.5 min
4.5 min * (480 NM / hour / 60 min) = 4.5 min * 8 NM/min = 36NM
This method isn’t 100% accurate, because you assume your initial speed and speed will change during the descent, but it’s so good an approximation that it’s used every day by real ATC to know when to start descending aircraft at which rate.