What I did was to work them out at lightening speed (I could answer as quickly as someone who'd memorised them and didn't have any sensation of cognitive load), along tracks so well-worn that you might say I'd memorised the fastest way to work out the fact! I think I had just gone through a process of working out the same small sum over and over again, as part of larger problems etc., until I could do it that fast. I had and have never found this to be a problem.
I was poking around in Aimee Yermish's blog the other day, and she had the story of a kid who couldn't memorize the quadratic formula no matter how hard he tried (due to 2e issues), but could derive it in 30 seconds - so the first 30 seconds of a test he'd spend working it out, and then be fine.
