I want to put a shed in the yard. The instructions say check level grade of site. When I use a string level the string is at one hight on the first stake and ends up at a differant hight when I come back to the starting stake. I want to know if there is more than 6" in the geade differance. Is there supposed to be a differance when the string returns as to when I started and does it make a differance as to how high I start the string from the ground.