You have 3 excellent answers, already!. Let me add a piece to Gail's answer about running comparable sales: 1/2 mile is a goog range if you are in a population center, but 5 or even 10 miles may be needed to show any 'comps' if you are in a more rural area. In some rural areas, the best comps may be on opposite sides of a town or small city (workplace and shopping hub) from the subject property but with similar neighborhoods and similar distance to that hub. A Realtor with experience in your area has accumulated years of market research and a feel for the curent market that can be invaluable. Call one up and make a link for your benefit!
Hope that helps. Blessings to you.
The market value of a property is what it is worth to the buyer...you, but not over what it will appraise for.
Do you have a Buyer's agent you can ask for guidance? There is no magical percentage. Offers in our area that are within 5% of list price have a good chance of being accepted. That's not to say offers within 10% of list price do not get accepted. It depends on how reasonably priced the house is to begin with. If the house is very over-priced for the market or has been sitting stagnant with no activity for a long time you with have more negotiating power and may want to offer an even lower percentage of list price. Conversely, if it is a fresh new listing with lots of showings, the sellers aren't likely to entertain an offer that is 10% or more below list.
There is no "standard" per se.
Each situation is uniquely different, and you will want your buyer's agent to research the comps in the area to see what the property should be priced at. Once you have that number, I would bid a bit lower than that as your opening offer.
That said, your agent should know how best to address that.