Depends how badly you want the house.
If you want to purchase the house today,
you can make a cash offer with no conditions,
the best offer any home seller could ask for.
If you think it is overvalued or that it might be on the market for a while,
you can wait and take your chances or make a stink bid at 15% below the offer price.
But before you come up with a number do some comparative research and see what similar homes in the neighborhood recently sold for in the last 3 months.
That gives you a baseline.