I used to have a chrono with 3 sky screens. It wanted to be 15' off the muzzle. This is how poor people set one up. Get some cotton string about 20'. Tie a loop knot on one end. Loop it under your scope ring and through other end of line. Pull that tight. Hold it along barrel and have someone measure whatever distance you need off you barrel, only have to do this once. Mark this with pen or tie a knot there. When you set up crono just pull sting tight and sight along barrel. One guy can set it up every time very quickly. Of course you need to keep the string for future use.