The maths...
Example 1. G/S 150 kts = 912000 ft/hr (roughly) = 15200 ft/min. That is your horizontal speed.
To achieve 3 degree profile... which is 5 percent... which is 1 in 20, you need 760 ft/min vertical speed.
Or done the quick way, 150 * 5 = 750 fpm.
Example 2. G/S 100 kts = 608000 ft/hr (roughly) = 10133 ft/min.
Again, to achieve 3 degree profile, 5 percent, 1 in 20..., you need 506 ft/min vertical speed.
Or in your head quickly... 100 * 5 = 500 fpm.
Never heard of applying an extra 50 fpm to the calculation, so that's my answer in an interview scenario... and I couldnt fly that accurately if I tried anyway. Gotta use this one as a rough guide for a ballpark figure... I got other things I got to look at in my scan!
Now surely there must be more useful questions that people have been asked in interviews during their careers???