most digital hygrometers have a variance of +/- 5%... it doesn't need to be exact, but more a 'ballpark' type idea...
if your eggs were too wet, then lower the humidity from what it read during incubation and go from there.
I dry incubate all my eggs (around 20-25% max, tho right now it's about 18%) and then jump it up to 60+ during lockdown. last hatch i kept it closer to 75% because i had to open the hatcher repeatedly to assist a chick that was upside down, and everyone came out just fine.
i highly recommend trying a dry hatch to see how things go. i've drowned plenty of chicks in the past not knowing that i kept them too wet, but not having the internet 15-20 years ago, i didn't have anyone to tell me differently...
as for calibrating the hygrometer, if you determined it's about 6 off, then make note of it and go from there. like i said, it's more of a guide than "it has to be this or that". so if it was too wet, cut it back next time. but i wouldn't cut back for lockdown, the NEED the humidity then or they'll stick to the membranes and fail to hatch.