See the following website : Naismiths Rule For Height Gain or from WikipediaOriginally Posted by Naismith's rule
Here is an equivalent formula for fell runners from:
Running uphill: energy needs and Naismith's Rule
In this note we make two points, from which follows a suggestion for running uphill: .... (2) corresponding to Naismith's rule of 1 to 8, in whole numbers, for walking or running in mountainous country (1 foot of ascent is equivalent to 8 feet on the level), the rule for running on a treadmill is 1 to 3 and the rule for running on roads is 1 to 4 or 5.
This result suggests that, if possible, a runner should zigzag up slopes steeper than 1 in 8 on the fells and zigzag up slopes greater than 1 in 4 or 5 on roads.