Not quite, it's a little more complex then that, if the elevation change is 500' and the distance you travelled is 5000' you have two sides of a triangle, but you need the third side, to do the calculation, so our old friend from school Pythagoras, you have side A & C, but you need A & B if a²+b²=c² then c²-a²=b² so 5000²=25,000,000 and 500² is 250,000 then b should be square root of 24,750,000 or 4,974.937185533 / 500 would be a 9.9% slope, not a 10% slope. The steeper the climb of course, the bigger the difference. Few hills are the same slope all the way up, so the best you can get is the average.