I suppose you talk about a device that measure humidity from humidity machines? Or is it a built in tester inside the machine? Supposing it is a external measure I see 2 solutions, neither of them perfect:
1: Have 3 (or preferebly more) controlled enviroments where you know the humidity, proceed as with any MSA.
2: have a second measurement tool that you know to be right, create the humidity needed acording to this tool, measure, then increase/decrease until you have for the next measurement.. Would take a loing time and you would, as I said, need to have a second tool that you could trust.
But most likely I missunderstood the question and made a fool out of myself.