Aha... be careful when talking about what specific value to use - as doing so may send us spiraling into the great debate over how kb/mb/gb are calculated 
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Either way - the reason that your current status bar code doesn't set the graph exactly as you'd expect is likely do to some math that I'm assuming is taking place...
Consider your example: based on some quick trial and error - a status bar code using V=sbL/4700372992 goes over threshold (red) at exactly 4,747,376,722 bytes selected... with the resulting value of sbL/4700372992 being 1.010; while just a single byte less results in a value of 1.009. I expect that the code that controls this part of the status bar might only be using a 2-point unit of precision... which might be rather trivial when operating with GB or even MB values; but down at the "byte" level, and with such a relatively 'large' byte value, Opus would need to use a pretty crazy unit of precision to make the bar graph go over threshold at a byte accurate level... at least- that's what I assume is happening, or something close.
Anyhow, I think that maybe a good middle ground (as opposed to using some crazy unit of precision that might still not work if some fool were to express a 2 TB drive in terms of bytes ) might be for Opus to provide an additional way of triggering the bar graph to go over threshold. I think many people like Orbidia are less interested in using the bar graph to see a "percentage" expression for which a "ballpark" value will usually suffice, so much as a measure of space remaining against a hard value... maybe some additional control codes along with some additional text and even some calculation capabilities would make more sense for such use cases... something like:
{wtree}{h!}<#ff0000>{hi} hidden</#> {i:/jtools\icons\sep.ico} {h!}{i:dir} <#0000ff>{sd}/{td} folders</#> {i:/jtools\icons\sep.ico} {i:.txt} {sf}/{tf} files
{bg+c1=#408000,c2=#ffff00,c3=#ff000,t=no,g=2,f=2,v=sbL-{$v=4700372992}} <#408000>{sbLa} selected</#>{h+}<#ff0000>, {{$v} - {sbLa}} remaining...</#>{h+} {rpad}
{bg+c1=#408000,c2=#ffff00,c3=#ff000,t=yes,g=2,f=2} <#408000>{df} free</#>
The exact syntax involved not-withstanding - I guess I'm suggesting that some additional use of V in the {bg} code be allowed such that you could:
a) set a hard or calculated value in {bg,V=) by which the graph would go over threshold...
b) use that value for other calculations which you could use to provide additional status bar messages...
In the pseudo-example above, I'm thinking that that if the number of selected bytes in sbL is greater than a status bar variable {$v} set to = 4700372992, then set the graph over threshold... then also use some sort of hidden string control that will hide the contents if the section is <= to zero (I guess {h+} makes sense for that) to provide an additional message that uses a value calculated from the hard value and the selected file sizes to message how much space remains before you'll go over threshold.
This sort of mod to the staus bar codes would seem much more useful for the typical use-case of determining how much data you could select in order to fill a DVD to the brim...?
Dunno - just thinking "out loud".