It doesn’t matter all that much, because as @EmeraldEyes said, above, the figure quoted is just a maximum and you don’t have to use all that, anyway. The important question, therefore, isn’t “What leverage is best?”. It’s “What MAXIMUM leverage is best (whether you use it all or not)?”.
The point: if the broker will allow its customers to use more than 1:30 (or 1:50 in America), then that proves it can’t be a properly regulated broker, so you should never trade there at all, with any leverage.
In that sense only, the cut off figure is 1:30 (or 1:50 in America). But that’s actually the only important “sense” anyway.