If a hygrometer is off by say 2% at 69% RH will that 2% difference be the same throughout the range or is it linear?

I have 65% RH Boveda packs but haven't opened them as they won't fit in the Tupperware. I'm still using my Drymistat with Xikar 70% solution, but I do have a sling psychrometer to read RH and temp from my A/C days. I've considered trying that to see how accurate it is indoors, but I'm not sure how it would relate to the solution kept in my fermentation chamber that is set at 64*.