Anywhere below 50 is fine.
Actually even at just over 30% of the network there is a decent chance to get six blocks in a row (the criteria for a 100% successful double spend attack). So we do not want to get anywhere near 50% or risk being seen as a danger to the network. Having 51% simply guarantees a successful attack. Someone correct me if I am wrong.
STOP IT! Blocks in a row is necessary but not sufficient for a 50% attack. You need more than just 6 blocks in a row to successfully 51% the network.