hey rutterbush, i have two tricks i do for right angle bends. if using 1/2" square, 11" long, I would put a center punch mark at 10 3/4". put it exactly in the middle of the stock. then make the right angle bend with the center punch mark on the side. It should be lined up in the middle of an imaginary diagonal line drawn from the inside corner to the outside corner. this should give you 11" from outside corner to bottom. the idea is subtracting half of the width of the starting stock from the total length needed. check out "elementary forge practice" by John Lord Bacon. It has some great ideas on forging.
the second trick i do is to forge a corner with plenty of stock to spare and then cut the piece to length. is that cheating?