First, are you sure there is only one mortgage on the property? If there is a second mortgage, especially if owned by another lender, the chances of successfully purchasing a short sale at any price drops significantly.
As far as how much under loan amount you can purchase a property for...the loan amount has no bearing on the short sale decision. The lenders will order appriasals or BPO's to determine what the market value of the home may be. They then decide whether they will recover more money by foreclosing on the property and selling it themselves (expensive and time consuming process) or if allowing the short sale will net a higher $ amount for them.
There is no formula for what they will accept under market value and it differs from bank to bank and from loss mitigation manager to loss mitigation manager within the bank. To add to that the chances of even being able to purchase a short sale at full appraised price aren't that great, as the banks are so backed up with the things that it can take them 3-6 months to approve or dissapprove the sale. By that time most buyers have walked, or the foreclosure department has already gone through with the foreclosure sale.
If your looking for a great deal, I'd suggest looking at properties the banks already own. Unless you are willing to wait a long time, and can deal with the dissappointment of not getting the property after such a wait, don't mess with short sales. I'd say 1 out of 4 short sales actually go through, and if the listing agent knows what they are doing, maybe 1 out of every 2 or 3 go through. I'm not saying don't consider them altogether, just know what your getting into before considering them.