I asked this question about 2 months ago in a different way and received much the same advice. Generally they said offer "There are too many variables, offer what's it worth to you". Not much help when looking at a J-37c that needed some money put into it after purchase. I have found that BUC and NADA to be quite worthless. Boats actually sell for well above their values. The BoatUS lady seems to do a better job valuing a boat. BUT I haven't actually had an offer accepted that was made based upon her input.
It would be nice to have some rational way of coming up with an offer.