I don't think we're technically supposed to share information on late fees between facilities, but I will say that any fee you charge should be able to be justified in front of a judge. Try to quantify the liquidated damages a late payment would cause your business in the form of lost revenue, outreach attempts, postage, etc. and you should be able to reach a fairly reasonable number.
In Colorado, we have a late fee safe harbor law of $20 or 20%, whichever is greater. That sounds pretty reasonable to me!