Turkish
Quite a complicated formula for RoD, and no allowance for wind.
Try the simpler one, RoD (Vertical Speed) = 5 x DME/GPS ground speed. i.e 120 kts Ground Speed x 5 = 600 fpm.
Old method
If you can't multiply, it gets easier, half the ground speed, add a zero, i.e. 120 kts GS, half is 60, add a zero = 600 fpm.
Newer method
Your method works too, but with mine,I also get time to pick my nose etc without losing the glideslope.
And yes, 300 ft / nm is correct, 7nm = 2,100 ft AGL for a sea level airport, or add 2,100 to strip elevation to get height AMSL for non sea level strips.
Gotta go, time to change hands.