Determining The RMS Value Of A Bus

NO -20 db as ref level for a premastered mix is perfectly fine. It just means though if you want to end up at -9 dB you are going to have add 11 dB of rms gain to your track. Which is quite a lot but
 
 
Here are some ways of doing it though:
 
You do not master at the time of the mix. Export the mix as a premastered file. Wait a week and master this separate premastered stereo file in a separate session. Any advice contrary to this is simply incorrect. Period. Mixing into some slight two buss compression is not mastering and quite acceptable. In fact if I know I am aiming for a loud master I often do it. It can help a lot.
 
-10 dB rms for pop music is not loud at all. Average mastered pop levels are closer to -7 dB rms. I agree that -10 or -12 dB rms would be nice but it is not that way in reality. (for Hip Hop even -6db rms is common)
 
In the following I am referring to rms levels. Peak levels should be up around between -0.1 to -0.2 dB FS. (in your final master that is) VU meters (real or VST) make it much easier to read rms levels.
 
Boost 11 limiter is one of the worst limiters on the planet so you will never get what you are looking for with that one. Get rid of it.
 
Obtaining loud masters is a series of processes and each process should not be drastic in nature but smaller. There are some simple steps one can do to obtain loud masters.
 
1  The Pre mastered track needs to put into an editor and the average rms level assessed. Often the peaks are high in the premastered track and the rms levels are a little low. Peak limiting your unmastered track to say -3db and then raising the gain of the entire track 2 dB will increase the rms level by 2 dB with little change to the dynamics. Sometimes a premastered track might be peaking up to -1 dB but have an average rms level of -20 dB. In these cases this editing approach is even more important. I might then limit the peaks to -7dB FS and add 6dB of gain to the whole track. The track will get 6 db louder and still retain lots of dynamics. Now you have just added 6 dB of gain saving a whole lot of gain adding later. Pre-preparation in a decent editor is almost vital before mastering actually begins.
 
2  The EQ is there to redefine your overall EQ. You can sometimes add 1 dB or so to the overall level after the EQ. Check the level going IN and coming out of the EQ
 
3 Compression only needs to be slight eg 2 to 3 dB gain reduction, slowish attack to let transients through. But the good news is you can add some makeup gain here usually about 2 dB
 
At this point if your tracks started at rms of say -15 dB you have added 5 to 6 dB of rms gain so you are in the ball park now even before the limiter. Even if you have added only 4 dB you are now at say -11 db rms.
 
4  The limiter should only be adding 3 to 4 dB max of rms. I use PSP Xenon. It is probably one of the best limiters on the planet and it can add 4dB very cleanly without destroying your mix. It is expensive but there are other limiters out there that could add say 3 dB of rms value to your track and cost less and not destroy your mix.
 
Now you should be up to -7dB rms and still sounding loud and punchy. One stage alone will not achieve this, it is a series of things that you have to do and a little here and there but it all adds up.
 
Some Extra Tips
 
Be careful of the multiband compressor. You have to get all the bands gain reducing the same amount otherwise the EQ of the track can be easily thrown out. I am not a fan of multiband compressors. Some say they are good for solving problems with a difficult mix. If your mix is great then you don't necessarily need it. I prefer full range single band compressors and this is one of the best I have heard and it is free:
 
http://www.tokyodawn.net/tdr-feedback-compressor-2/
 
Very important. Limiters (and Xenon included) do not like it when the bass end is out of control. Out of control bass will smash the limiter first and destroy your mix fast! You must get the bottom end of your mix perfect and it all starts right back at the LP64 EQ. Watch the GR meter in the limiter and see what part of the music is kicking it down hard. If it's the bass then your bass end is wrong. Sort it then check again. You will find when the limiter is responding to all of the music equally you are in much better place.  You can then easily add 3 or 4 dB rms to your track without any distortion or breakup.
 
PSP Xenon is on special right now too and it is a good time to invest in it if you can. Danny has made a good point too in that Xenon does not always limit certain styles the best. He also uses the Waves L3 (L2?)  I think and he points out that it does a slightly better job on certain genres. (Power rock, heavy metal, more crunchy styles.) I don't doubt him. I would like the L3 as my second limiter choice too but it is also not cheap either. I think Xenon can handle a wide variety of styles though and well.