Ever since I suffered a stroke last year, my eye sight has been a bit squify, so I find greatly helpful to use a grid over the subject and the same proportional grid on the paper. Recently, when telling one of my old arty farty acquaintances, she said it was cheating, I told her she was being absurd and the method is at least as old as Durer, there are a number of his works showing drawing grids and perspective aids. If it was good enough for him... Woodcut, The Draughtsman, Albrecht Durer