Sorry, but need to ask a simple question. I understand why one strings a line to get straight reference lines on the side of the strongback (the 4" side) but why cant one measure to the center on the top (2"side)? The side usually is crowned up or down but the top seems like to string a line if the length is cockeyed and twisted then the line will be too. Even if one were to lay it on the floor the floor may be out of level at such a long length as well. Why not just measure to the center...wouldnt that be even more exact? am I missing something here? Thanx guys, Andy