I like to make it about 2.4 BB.
A lot of bad players will call in the big blind with a very wide range, giving a bit too much guesswork post-flop for my liking.
Also, I think the min-raise will get 3-bet much wider than a slightly larger one.
In both cases I think the improved hand definition, plus the increased fold equity, are worth the extra investment.
I agree that most online pros go for min-raise or tiny amounts above it, but I have seen some top players recommending a bit more, so I think it is a matter of which works for you.