That's what I do. Do a calculation from a 3RM to get the estimated 1RM. Then I drop 10% from that for the month. I think they run a rough test in the middle where you either increase it by another 5%, stay steady for the month, or decrease it 5%. Once you're on the 3rd consecutive day, I seem to know if I'm using the right number or if it's too easy or too hard.