Building ramp for shed
As Ironlight said, the angle is going to be a function of the type of mower that you will buy, and the clearance that the deck provides, so stop my a few lawn and garden centers, take some dimensions, and sketch it out. A couple of thoughts though; if these ramps will be permanently positioned, have you considered earth, with grass seed? If not permanent, how about a set of light weight aluminum ramps for loading things in a pickup or trailer? Probably less cost than posts, planks, etc., and easy to stand against the wall, inside, when not in use.