A great humidity level for any growing environment is between 55-65%. Anything higher than 65% relative humidity increases your risk for fungal diseases like powdery mildew. Unless you’re running lower overall temperatures, anything below 55% will increase VPD too much. Keep the humidity levels in your growing environment the same, both with the lights on and off.
All humidity sensors can have a substantial error factors, so having more than one to base decisions off of is very important. Use the green portion of the Vapor Pressure Deficit chart below to match the appropriate temperature and humidity levels: