I've observed that there is no "percentage" you reduce a hopeful full listed price for, to arrive at its actual value.
For example, if a boat/car/house/whatever is advertised at £120k, its not "20% wiggle room" and actually worth £100k. It could be worth £120k (or even more), or it could be worth £50k and the seller is a dreamer. Also, distance between sale offered price and actual value doesn't necessarily mean a correction will occur, some people just don't need to sell (or are too stubborn to adjust the price) and it will remain unsold, possibly for a long time.