I might have missed it... but I always setup string and square that up before digging any holes. As for squaring up, (1x) 3,4,5; (x2) 6,8,10; (3x) 9,12,15; (4x) 12,16,20; (5x) 15,20,25 The longest you can do the more accurate I think it is.
You can still do the string thing at this point you.
View attachment 450174
You can still do the string thing at this point you.
View attachment 450174