Have you had previous experience with this incubator and similar eggs? If so, were those early or late? Let that guide you.
If you don't have previous experience, it is a bit of a crap shoot. There are different reasons to stop turning and raise the humidity. For most of those reasons, a day early or late won't really matter. I don't think the actual time to start lockdown is extremely important except that the key is to get the humidity up before eggs start pipping. If they start pipping before you raise the humidity, there is a chance (not a guarantee, a chance) that they could become shrink-wrapped. If I had no history to go by and had to make that decision, I'd go on the 17th day.