Is there a secret formula?

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I have read a lot in these forums about pre-FC'd homes. I would like to know, however, how one goes about choosing the right pre-fc home. For example: Realtytrac has a great data base for pre-FC'd homes, but what makes one home better than the next. Im not talking about the physical features of the actual property, but rather about the numbers. Is there a formula or calculation that I can use to determine why that is the home I need to pursue?

Example:
Amount of equity in the home: 100k
Amount left on the loan still owed to the bank: 150k
Market value (determined by Appraiser and CMA): 300k

What do I want and what do I not want? What's ideal? What percentage do I need to look for to find a "winner"?

Thanks