Don't know how to answer your question withan actual number, but the walking away part is critical. Also have an maximum limit and stick to it. Don't get caught up in the "What's another $100, $1000, etc. when I am spending X dollars" philosophy. I call this "Boat Price Creep" or "Ebay Bidding-itus". Pick a number and don't budge from it.
FWIW, my wife and I made an offer on a boat that we are intersted in. The original price was X dollars. The price dropped recently to 88% of X dollars. We offered about 73% of the original asking price or 80% of the reduced price. The offer was final and firm, as in I won't up my offer if a counteroffer comes.
I have been instructed by many people that have bought and sold boats, that your intitial offer should be 65 to 75% of the asking and pay no more than 80% of the asking price. For this particular boat, I have seen it listed for as much as $15K more on YachtWorld.com for a similarily equipped boat. So, if our offer is accepted and the boat passes the survery, I feel that we got a good deal.