Just curious, if a hill was 60 feet higher on the way back, didnt you encounter that hill on the way there? I wrote this on another thread, you can use google earth to view your location before flying, scroll over points in the direction you intend to fly and check what the change of elevation is where ever the cursor is. if your elevation is a 100ft where you are taking off and you are moving it the direction you plan to fly at 150ft and the number gets lower say like 60ft it means the ground is higher and when you fly over it, you are going to be 110ft ft AGL in that area, if the number gets higher 200, it mean the ground is lower at that point, and you will be 350ft AGL Do a lil math and you can set your setting accordingly.
you RTH height is from the Home Point from which you take off, what ever that elevation will be. But i do the google elevation check, if i think i may be flying in another unfamiliar area, you can observe the elevatioon change in the lower right corner when you have the map pulled up, just view what obsticles are in the area as well, scroll over them too. the elevation should constantly be changing.