I just always imagine how many people would fit laying down the length of the jump, probably about 6 feet times how many people would fit. Obviously this isn't dead on but works well and you don't have to bring a tape measure to the hill, i guess you could use your ski if you really wanted to be exact. Pretty much though thats how i eye up stuff like cliffs or jumps, hell sometimes you'll even spot someone actually laying on it while your trying to eye it up.