A simple way to set up a 90deg angle when working out bases for patios, decking, sheds etc. etc is 3,4,5. The two sides of a right angled triangle should be equal to 3 and 4 (mm, inches, miles whatever) if the hypotenuse is 5 you've got a dead on right angled triangle, hence a dead square corner (Why am I typing this ? the alternative is to watch cbeebies 
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