Just check for 90 degrees with a normal square, tack two bits of flat or something to check your pitch angle, what width is the overall width of the shed going to be?yeah to be honest thats my biggest concern, is getting the angles spot on on such big items, its not like you can flip things about on a bench and check it with a square! first time il realise its wrong is when its hanging 15ft in the air out of a crane