ok fair enough, so if you like maths, lets have a close look:
200kt*tan3°=A(rate of descent in NM/h)
A/60*1852/0,3048=1061ft/min
300kt=>1592ft/min
250kt=>1326ft/min
so if you like rocket science you use that, personnally i divide GS by 2
300kt=>1500ft/min
250kt=>1250ft/min
200kt=>1000ft/min
simple and useful rule of thumb, but anyway you and me are not so far from the truth
like a 3° path is exactly 318ft/nm 300 ft for me 330ft for you
Do you use mach number * 3= V/S for 3° path without too much wind and high mach number
mach number ruffely *10= nm/ min