I am building retaining walls and a couple of them will connect to my boundary wall. Obviously I want the two to be at right angles. I am using Pythagoras to attempt to get the two walls at right angles but ... The length of the end of the wall from the boundary wall is 450cm and I have used string to produce the right angled triangle. Needless to say I am not confident that this has worked properly as the string stretches. I have also used a piece of wood 200cm long to provide a more 'stable' measurement and then extended another 250cm from the measured point. My mark one eyeball says that it looks ok. I do not have a 10 metre metal tape measure. I have even calculated the likely error. ie if opposite and hypoteneuse are in error by 10cm (one longer one shorter) then the angles only changes by less than two degrees and the end point will be out by 7cm. Am I being anal? Should I trust the eyeball? Is there a guaranteed method which does not involve me buying items which will never be used again? Any non-abusive suggestions gratefully received.