There is no rule of thumb when making an offer. Houses that are priced "right" will sell very close to asking price (or above) and homes priced too high could have a large difference between closing and asking price.
Prices are determined by market comparable sales. A local agent can get that information to you and best help you interpret it to make an offer and understand the local market.
If you would like information on closed sales, you can order a market snapshot from my website, below, which will give you an idea of where similar homes are selling. The data is taken from local MLS boards, fyi, so it is accurate. However, a seasoned agent is your best bet when interpreting that data for you.
Always remember, too, that in a buyers market (and again, this will totally depend on your local market conditions) not all the offers/numbers make sense and deals happen that are outside the comp range dependent on the sellers and buyers motivation.
Good luck with your offer.