Thanks JaeG - will doHere's an article about how to calibrate your hygrometer:
https://www.backyardchickens.com/articles/calibrating-hygrometer.50678/
I incubate quail eggs with high humidity the whole time. For me it works better but it all depends on your incubator and climate. I recently swapped some eggs putting some from my incubator under a sitting hen and putting the ones she'd been sitting on in the incubator. Technically I was returning her eggs to her (I wanted a good hatch rate from her eggs).
The ones she'd been on had huge air cells (despite the ambient humidity indoors having never fallen below 50%) and two pipped, one externally, one internally but didn't make it. I'm still waiting for one to hatch but it's taking forever. If they lose too much moisture they cannot rotate to unzip and I suspect they'd just lost too much moisture. The chicks that spent most of incubation in the incubator are also bigger than the ones naturally brooded.
As a side note the hen decided to take a break as her eggs were pipping (so much for 'lockdown'). 6 out of 7 hatched successfully. The one that didn't hatch hadn't even pipped internally (I think - she lays extremely dark shelled eggs which are almost impossible to candle).
Make sure you move your eggs around in the incubator as all incubators have warm and cool spots. Moving them around averages out the dips and highs.