An offer shouldn't be based on percentages. An offer should be based on what similar properties recently sold for. Within those numbers you will have the lowest sold price, the highest sold price, the average sold price and the median sold price. Your offer should be based upon those numbers and very rarely should it exceed the max sold price unless the house you are making an offer is better than the house that sold at max price (more beds, more baths, updated, etc). Once you compare the sold comparable properties then make your offer but don't just pick an offer out of your head based upon a random percentage.
A property priced well from the start may end up selling at 95% of the asking price. Whereas a property that was overpriced to begin with sold at 80% of the asking price. Your agent should know the market and be able to guide you accordingly. An educated buyer has its advantages.