It's a myth to think there's a formula to offer prices. If any realtor tells you, "oh, just take off X %," get a new agent.
Houses are worth what someone is willing to pay. You can determine this by seeing what comparable houses have been selling for. Have a realtor prepare a true apples to apples CMA to determine your offer price. Make sure the comps are of SOLD houses, not what's on the market. Who knows if any of those houses on the market will sell.
Also, make sure your realtor makes adjustments for different features and the condition of the home. If they just say, "well, this is the average," ask how specific things can affect that number. You can add some cushion to leave room for negotiating, but if the house hasn't been on the market long, and it looks priced right, and the comps sold in short periods of time, I'd factor that in to how much you choose to offer. You can submit a lowball offer, but if there are multiple offers then you'll miss out on the house you want.