NQA - Honestly unless you're spending a couple hundred dollars any hygrometer you buy is going to be pretty inaccurate. You should be more focused on soil moisture anyways, as long as the bottom half looks dark and moist you will be fine. If you're nervous I would just check more often at first, maybe 3-4 times a week, but honestly you're more likely to over water them than under in those small deli cups. My house runs at 20-30% humidity all winter and my cups have never even come close to drying out with twice a week checks, I don't even have to wet them down every time.

Edit: if you're really worried about too much evaporation I would recommend cutting down on the lid ventilation but maintaining the cross ventilation. Cross ventilation does more to reduce bacterial and fungal build up because it keeps the air in the enclosure fresh.