An approximate value for the required feet/minute can be calulated by taking the groundspeed in knots and multiplying it by the required % gradient.
So for example to get a 5% gradient at 100 knots ground speed requires 100 x 5 = 500 feet/minute ROC.
This is based on the fact that 1 knot = approximately 100 feet/minute.
1 knot is actually closer to 101.3 feet/per minute so a better approximation in the above example would be 506.5 feet/minute ROC. But 500 feet/minute is reasonable approximation.