Hi Tieg,
like any kind of tool, audio or otherwise, you have to apply it in the right way. There are two limiters in Logic (by labelling not including setting up the compressors to be a limiter) the limiter and adaptive limiter. By design it is supposed to stop signals from going over a certain threshold, however, no signal is generally the same!
So, in some applications, the rate of change of the audio signal might be so fast that the limiter has to be 'tuned' in order for it to behave as it is supposed to.
As a rule of thumb, it is always wise to set the threshold just below 0db.... as Rounik suggested, maybe -0.3, 0.4... really as long as it's a few points of a decimal place below 0, that'll be enough.... and then it's case of playing the knobs until it sounds like you want it to. Lookahead is there if you need it.. and like dropping the max ceiling to -0.3 or 0.4, it is there to give the limiter a helping hand for analysing the signal before it processes it. Using limiters with lookahead are therefore not recommended when recording or tracking MIDI performances because by it's nature, the lookahead delays the signal (additional latency) for it to limit it properly.
As for how much to squash your signal by, well that is purely a matter of taste and it a hot potato of a topic too! Some styles of music warrant less dynamic range. Some warrant more - think pop and classical respectively. There is a growing movement to standardise dynamic range though, like in the film industry, as there is the perception that louder is better.... but that's another thread!
Cheers,
j
