Vapour pressure deficit combines temperature and humidity into one number, which is more useful than tracking either alone. Aiming somewhere near 1.0 to 1.2 kPa through flower keeps transpiration steady, and the environment charts on https://kushindicaseeds.com convert those figures into plain temperature and humidity pairs. Cheap hygrometers drift badly and are worth replacing yearly. Getting this roughly right prevents a surprising number of leaf problems further down the line.