So I just read a post on this forum by Unterleigen, and he says that the MPB studies on keto to date have not actually compared 1% vs. 2% keto shampoo. For some reason I had it in my head that they did, but it would seem that only the 2% was studied (and found to have a mild positive effect). Let that be a lesson to me not to advise from memory.
Since the 1% hasn't been studied as an MPB remedy, we don't know whether it's as effective as 2% or not. I'm going to stay on the safe side and continue the 2% solution, but if you want to try the 1% and see how it goes, it's up to you.