I have explained all this many times and well before this guy with the K System posts I have done. -18 is a good ref for sure but I can see why -20 is actually better. It is aligned to the film score reference of -20 and also you get 20 db of peak headroom which is 2 db better then -18 dB. And if you are making -20 dB equal to 85 dB SPL in the room then the music can get as loud as 105dB. Fun.
Also I like the K system approach of 3 ref levels. And -14 is great too. Because you can recalibrate your system and just be creating a louder rms wave the whole time from the start tracking right through to final mix. 3 Ref levels are better than one. When you work at -14 you are already pretty loud rms wise and you have less to add in mastering to achieve a very loud master later. -12 is also an interesting level to work at too.
RMS meters in standard DAW's are not so good because they are indicating something low on a scale. A dB or two variation is not easily seen. On a VU at the 0 dB mark such a small deviation is clearly visible. You need a VU meter that is hitting 0dB Vu when the ref level is being played back. And better still a VU that matches the ballistics of a real one. There is so much information contained in
how the VU moves around.
I have got my computer monitor sitting directly on top of my hardware VU meter display. I can put the VST's at the bottom of the screen and see all of them at once.
The Klanghelm meters are still a winner for me. The Klanghelm meters rise and fall the same way the real ones do.
http://www.klanghelm.com/VUMT.html PSP has released a nice looking VU meter VST.
http://www.pspaudioware.com/plugins/tools_and_meters/psp_2meters/ The PSP rises the same but does not fall quite the same as the real meter. I have spent some time trying to get them to match. I may need to spend some more time to fine tune.
They can all be calibrated for any ref level and there are many parameters you can tweak which effect how they behave and move.