There is not one answer to your question....there are no percentages, rules, etc regarding how much more a buyer should offer in a situation like this.
What does your realtor say?
What did you offer in the first place?
And how does that compare with current market values of similar properties in the neighborhood?
Remember, even if you write an offer for above list price, if you're getting a loan, that property will have to appraise for what you offered in order for the loan to be approved.