Every time I have to do some weird number like 65% from my 1RM, I simply give up. That's why when I'm trying to reach my 1RM, I to to go for the tens (10, 20, 30... and so on).
I'm doing the 5x5 program and have no trouble doing the math when I start, somewhere around the 4th set of squats I usually start having trouble it only gets worse until I'm just using my whole rest cycle trying to figure out if that was the fourth or fifth set.