I am NOT the drying expert but those humidity levels seem pretty low to me. Your going to dry them out pretty quickly. The goal is to get a nice even drying where the inside is drying at the same rate as the outside of the bud. That said, when you notice the outsides getting dried, you may want to throw them in a jar or large bag to let the moisture level in the bud equalize for a day, then moving them to the rack again.
Also make sure your fan is not blowing directly on them. I just harvested 4 stalks early because I accidently moved my dehumidifier to close to my SCROG. It dried the shit out of the buds the hot air was blowing on in a single day. Threw the whole bud in a bag and 3-4 hours latter they are all nice and moist and ready to properly dry.