As the other posters have mentioned... short sale isn't about the seller anymore, it's about the lenders. My BIL short-sold his condo (long and really irritating story of his impulsivity) and not only did it take forever, but in some cases there is more than one "lender" involved so you have to get each debtor to agree to the "adjusted" amount.
Three months is nothing in this market, so don't factor that in. Personally, I would think that the bank is going to reject an offer that is 20%-30% ADDITIONAL loss for them- remember that they have already agreed to "eat the difference" and any further reductions in price hit them directly. Think about it... a person owes $150,000 to you and you say "okay, give me $100,000 and we'll call it "even" and they come back to you with an offer of $70,000. Would you take it if it meant that you were actually taking an $80,000 hit? Probably not.
I would come in with 10-15% below asking price, knowing that you will likely pay 5-10% below asking in the end.