Having now tried 3-4 different mixing programs, I use the one in Multideco and apply the necessary "Kentucky windage" for things like temperature and how much the He pressure increases, and how quickly. A friend remarked this year that two topoffs I did for him were "perfect," any my rejoinder was that I should have bought a lottery ticket that day.
More seriously, a couple-three percent of He either way won't move the needle nearly as much as a half percent of O2. Get the second one perfect, and eventually the first will fall into line. Sort of. On a good day. More or less, anyway, and more than good enough nearly all the time.
It's a lot like a perfect landing in a small airplane. Everyone walks away - good. It wasn't uncomfortable but there was more "thump" than you needed - better. It was pretty smooth - great! The only indication the wheels touched the ground was the tire squeal - awesome! Rare, but awesome.