I'm guessing that you're in the UK as that's a normal UK broadcast spec (it's the 18dB digital headroom that suggests so, most of the world uses 20dB).
OK then:
"zero level" aka line up tone (the 0dBm, although that's really a redundant term in digital audio nowadays) is at -18dBfs, i.e., 1kHz tone set to -18db in your workstation.
A PPM is like a VU meter, but with a very fast "rise" time and a slow decay, so it rides the peak levels rather than giving an average. Hardware PPMs are quite expensive but I use PPMulator+ (generally via Audio Hijack Pro on the Mac) and it does the job nicely.
http://products.zplane.de/index.php?page=ppmulator
Line up level is at 4 on the PPM and the units are 4dB apart, so PPM6 is 8dB above line up level. That's your maximum peak level. Usually speech is best with the meters peaking roughly midway between 5 and 6 and music about half a unit or a whole unit lower, because music tends to sound subjectively louder for the same peak level and PPMs aren't good for judging subjective loudness. (That depends somewhat on how you use EQ and compression, so do use your ears to make a judgement. Personally I like only a little compression and only use a limiter as a "legaliser" to catch any odd peaks that I've missed.)
Hope that helps.
