What is the ideal humidity in a grow tent at each stage?

Young plants have small roots and take in water through the leaves, so they like it humid. By late flowering, high humidity is the main reason for mould and flower rot, so 40 to 45% is safer. If you have trouble holding exact numbers, a steady range beats chasing a number every hour.

Use a humidifier when the air is too dry and a dehumidifier when it's too damp.