and always prepare to lose a deal if it turns out that you under bid. There are more on the market. On the other hand, don't regret even if you find out later indeed you could have got it cheaper by offering a lower bid. When situation warrants, be a little flexible to get the deal done - as long as it still is within your value bottom.
There is no such thing as "best" deal, but there are plenty of good deals.
15% or any set number is nonsense. It is not only location specific, but also case by case for a property. Too many factors come into play.
