Unfortunately there is no mathematical shortcut to determine what to offer. Some sellers put their house on the market for the exact amount of money they want to receive. This is especially true when a seller has already reduced the price of his or her home. Most sellers add a little wiggle room in the price to allow for negotiations, but the amount of wiggle room is not a set percentage.
Remember that when you make an offer on a house, three things can happen.
1. The seller can accept the offer.
2. The seller can reject the offer. Then you can make another offer.
3. The seller can make a counteroffer. This is the most common response.