I'm making a song, I decided to follow this little guide for getting the levels of each element right:

http://www.tweakheadz.com/perfect_mix.html

It says rhythm guitar should be at -15dB, but when I do that it's really quiet, seems much too quiet in comparison to the rest. Is this something to do with compression? Also, it doesn't mention lead guitar, how loud should that be in relation to the others on there?