I think Arsenal, City and Chelsea are safe. That leaves Manchester United to contend with 3 other clubs for the remaining place - Liverpool, Everton and Spurs (5,4 and 3 points ahead of United respectively).
Liverpool are playing better than us, have easier fixtures and are 5 points ahead, so it would take a super optimist to say we have as good a chance. Let's be super-optimistic and say we have. That leaves Spurs and Everton - both ahead of us at this point. Let's be niggardly and give Spurs a 12% chance of qualification and Everton 8%. That leaves 80% to be evenly divided between Liverpool and United - 40% each. So:
United - 40%
Liverpool - 40%
Spurs - 12%
Everton - 8%
Total - 100%
Personally I don't think Everton's chances are 5 times worse than ours, and Spurs have just beaten us at home. Also, I think Liverpool, at this point, are entitled to fancy their chances more than United. So 33%, or 1 chance in 3, might be more realistic.