There is no formula you can use for that. However, the asking price is usually the bank's internal estimate of what it should sell for. They may or may not go lower than that, especially if the market value is higher to begin with. Your Realtor should be advising you of their opinion of value and what they think the bank will take.
If you're not using a Realtor, you're probably at a disadvantage without the experience they offer. If you're using the bank's listing agent, remember that by law that agent works for the seller and will do things in the seller's best interest, not yours.