I'd say as long as the required run rate still hovers around 10/ov you're in with a good chance but there's the number of wickets lost to factor in too, and which batters are still in or to come.
So if one of your top 5 is still there and you've got some big hitters still to come, you've 8 overs left and 80 to get you're feeling confident, 100 to get you're ok, 120 then you need a couple of big performances.
Bearing in mind 20+ runs can be knocked off a total in one "big"over the equation can shift quite quickly. Similarly two or three quick wickets later on and you go from 4 to 7 batters out and you find yourself relying on bowlers who can bat rather than batters
Is this making any sense?
I should add I don't follow T20 nearly as closely as the Test matches so this isn't an expert opinion at all