Rate of
Descent (RoD) for a 3° GP = 5 x Ground Speed
5 x 120 kts = 600 fpm
For anything other than 3°:
Rate of Descent (RoD) = 5 x Ground Speed x (GP Angle / 3)
Or use the CRP-5:
Place 1:60 Index against Glide Path angle.
Read Ground Speed on Inner Scale and corresponding RoD on Outer Scale.
If you are given a Gradient instead of a GP angle then place the Red Arrow at
1.0 against the Gradient percentage.
Again, read Ground Speed on Inner Scale and corresponding RoD on Outer Scale.