Here is one technique, based on the "miles per minute".
For one nm per minute (60kt), the rate of descent corresponding to 1° is 100ft/min.
Example : at 150kt, you're at 2.5nm/min. So the rate of descent corresponding to 3.5° is 2.5*3.5*100 = 875ft/min
The exact figure is 927fpm, that is a 6% error.
This is useful because you can fly at multiples from 60kt.
eg, when in a descent at 420kt, one degree is 700ft/min (6% more in reality but who cares, the VS indicator does not differentiate between 700 and 742ft per min)