31% on the front end (31% of your gross income as an allowable debt towards housing expenses) and 43% on the back end (43% of gross income divided by your housing and other credit related expenses (Child support, car payments, credit card payments, etc.. Not gas, food clothing, etc..) is the amount that the industry says they will allow you to go up to (Higher in some circumstances). That may be too high for your confort level, may not but that is the guideline they use to qualify you.
FICO is another story. How low is too low? It all depends. How long ago was your short sale? What is your FICO now (Don't use the fico score from myfico.com or some free credit report, they aren't the same)?