There's really a lot more that goes into determining what to offer on a property other than that the property is in pre-foreclosure and is on the MLS.
The bank holding the loan(s) has done what's called a BPO or broker's price opinion to give them an idea of the value of the property and to help them determine what a reasonable sales price will be. The bank is looking at comparable properties in the neighborhood just like any seller would.
There's no formula - it's got to be a case by case process. It's unlikely that going with a flat 20% off the price is going to be a successful strategy.