What Level should Peaks hit on a Mastered Track ?

I'll assume you mean RMS, because if you're peaking over 0dB you've got bigger issues than distortion .

There's nothing set in stone, I try to avoid the loundess wars and focus more on quality if at all possible and dynamic music can have a heavier impact on how far you can push. Usually I aim for around -12 RMS  and a decent lookahead brick wall usually keeps it around that, with maximum peak limiting being around -0.3 to take into account intersample peaks.

At that level it translates fine with with little to no distortion.

I didn't mean RMS , but the peaks .

The output of a limiter can be adjusted so you don't hit right at zero , but instead something like -0.1 . I read that you should aim for -0.6 if your gonna convert to MP3 . Is that true ?

It's been years since I've bought a major label record that the peak wasn't 0dbfs....and "true peak" +.8-2....that said, it also sounds like ass. It's why digital attenuation pre DAC (or on the DAC chip itself) is important to modern masters sounding OK.

I always use -.5dbfs for my peak normalize for full scale export. I do that because I did -.4 for years but at -.5 I've never been able to have an encoder or anything go over 0 for ISP or "true peak" (reconstruction estimation)....-.4 occassionally can register one--at least in whatever reconstruction modeling they do to detect ISPs....

Well as it turns out the Shadow really does know ............

A lot of Pro CD's like popmann has said are hitting 0 .

I'll be safe and go for -0.5 .

Ok for my genres of rap trap rnb pop top 40 edm I use ozone 5/ FF proL / limiter no.6 with InterSample peak protection enabled, and usually aim for -.5 for MFiT / aac / MP3 encoding. Sometimes -.8dbfs is necessary. I find that 320 Kbp/s constant bit rate sounds better to me than Variable bit rate at 320, which also sounds better than MFiT which 256 Kbp/s VBR AAC. My rms levels may be absurd to some but -9 rms ( dbfs ) to -5 rms. Last mix and master I posted via the de esser thread the rough mix was DR3, with a clipped beat of DR4 and had peaks +3-+4 over O dbfs...

When the client pays the client gets what the client wants...
When it comes to my music, productions, mixes and masters, I am more conservative for the genres, DR9-DR6.
Mixes peak around -8 to - 6dbfs. I can normalize to peak level, say -.5 dbfs and use a limiter set to prevent ISP if that print or bounce is for a lossy destination, and I think that keeps dynamics and usually is super loud. I also prepare for that loudness level from the beginning of the production. If I feel or client wants louder after normalizing to -.5 dbfs, I can use Pro L add 2-3 db ( - 1 to - 2db GR ) and the next thing we are at DR5 and less, which on most consumers DAC past 80-85% volume will cause distortion...

The irony of spending tens of thousands on equipment with the lowest noise floors and highest dynamic ranges to squish the end product into a distorted brick.