I am prone to stick a Universal Audio Precision Meter on my Master Bus at times, although what exists in a DAW should be in theory be the correct reading, no? Or do you professionals use the meters from an outboard desk to manage levels. 
 
In my headphone amplifier set-up when gauging it, I found -18dB to the best setting on the monitor volume for cross referencing the actual measured value.
 

Sonar show rms levels as being 3 db lower than what they should be which is a bit weird.  Normally rms is 3db down from peak and Sonar is doing that correctly.  But the standard for rms readings is that the rms reading of a continuous sine wave should be the actual peak value of that sine wave, not 3 db down.
 
The main issue is that the DAW rms meter reading is way down on the scale so you are not getting the full scale reading that a VU meter has around 0 dB VU.  Some DAW's (eg Studio One) allow the bar graph meters to go into K System mode so then they jump up to around 0 db VU and all the headroom is above that now.  It is not a bad indicator but it still misses out on the ballistics concept though.
 
Yes I use real VU's all the time.  But that is not an option for many.  It requires the purchase of decent VU meters and they are not cheap.  The proper electronics to drive them too.  And you have to mount them in a box etc so it is all a bit fiddly.  You can buy them too fully made up in a box etc but they will be expensive.
 
The real VU's are something else again though.  Hard to explain it.  They dance in the ultimate way.
 
Hey this is cheap:
 
http://www.ebay.com.au/itm/Custom-VU-Meters-3-Large-Simpson-VU-Meters-/111650225948?pt=LH_DefaultDomain_15&hash=item19fedf3f1c
 
Problem is it is in Australia just around the corner from me. I should check this out myself! Except it is US voltage eg 120V so it would be perfect for you guys.

Hey Jeff, any thoughts on these?
 
http://www.amazon.co.uk/N...&keywords=vu+meter

 
I don't see the size of those meters, but I wouldn't expect much.  And you'll need the circuitry.
 
I'd go with something more like these :  Simpson VU meters
 
I've caught some pretty nice NOS meters for great prices if I waited for the right moment.
 
You can also remove them from old equipment that you can get for cheap.
 
EDIT :  Hey, did one of us just grab those meters Jeff linked to ?  Jeff ?
 

The actual VU meter itself is pretty important.  I prefer the larger front sized VU movement.  It means the needle is longer and there must be some change in inertia for the needle.  It would move or dance differently as a result.  It will cost anything from $75 up.  And you need two of course.
 
Here is a link to the type of meter I like:
 
http://www.ebay.com.au/it...722&_qi=RTM2010733
 
That particular ad might be for one meter only as well.
 
The side movement that Paul P has indicated is interesting.  I have not spent time looking at that type of VU meter at all.  It may be excellent I would imagine due to the needle being made longer.  They can be harder to read a little further away.  The needle gets disguised into the scale markings.  I still think a flat panel meter gives away more information faster.  As to where the needle is what it is doing.
 
The Amazon meters that Bristol refers to look OK too but smaller.  They tend to have a slightly different ballistic.  A larger meter is easier to see further away.
 

Thanks Jeff & Paul

http://www.bluecataudio.com/Products/Product_DPeakMeterPro/
 
Interesting plug-in and not too expensive. I've never used it however it looks like a good tool for the job.

I have the Blue Cat meter and it is great to have and tells you a lot. I don't use it much now though. I have found it is good to work in conjunction with this too:
 
http://www.pspaudioware.com/plugins/tools_and_meters/psp_triplemeter/
 
This covers all the VU movement that the PSP meter does not have.
 
The free Orban Loudness meter is good too.
 
http://www.orban.com/meter/
 
 

I have a couple of Universal Audio plug-ins that use K-Meters however only the peak VU, zoom scaling and K-Metering Scale which only gives you one view of the dynamics. I tend to use the drop-off volume on the release of transients as a guide to where the average is at the moment. 
 
It gets harder to judge the more complex the mix gets and at times have found that I've been a 2dB from the desired RMS after rendering. Leaving me to believe I'm reading the meters wrong or something else is amiss. 
 

I like to change K ref levels.  I like working between K-14 and K-20.  I think there is an art to reading a VU meter.  The real ones seem to make a fraction easier.
 
When I am doing a mixdown of a pre master I will go for a total reading on my VU's so the needle is just reaching 0 dB VU for most of the mix.  Right now I am doing a mix for guy and I have decided to make K-14 my ref level on this.
 
The VU needle will read low from  0 dB VU and sometimes goes a little over but not wildly go over at anytime. There is some point where the needle tends to hover and be a lot of the time.  That is the point to get around
0 dB VU.  After I replay this I get an identical reading back and there should be no reason why it should not.
 
A bit of the skill is in getting the needle to averaging around this 0 dB point.  You may be setting it a little low if there is wild swinging.  You tend to limit the upper level of wild swinging to around 0 dB VU instead.  But then the average level ends up a little low.
 
When a mix is doing well the ballistics of the needle seem to be a certain way where it just hits 0 dB VU and not go much over but the transients are still present and punchy.  Then you can creep the rms level of that needle movement to being right on 0 dB VU.  (you can practice looking at great mixes on VU's such as Steely Dan's 'Everything Must Go' CD)
 
Wild swinging of the VU at mixdown means something is responsible and it usually only one thing as well.  Just have to track it down.  Do some editing, reset any dynamics processing on that track and you are away.  The wild swinging in the main mix has gone now.  Now the VU is showing much more of average level and you can adjust it now to sit right up at 0 dB VU.  You can easily add 3 to 4 dB of rms gain once wild swinging of the VU is under control in a mix.