...how do I know when I've set the volume on the monitors to reflect accurately the volume of the master output.
Get a Radio Shack SPL meter...digital or analog. I kinda prefer the analog since you can set the VU needle to fast or slow movement, and it's easier to see the "average " travel of the needle than when you're watching digital number flipping around....but that's a personal choice.
The RS meters have A and C weighted modes....use the C.
Put the meter on a stand or something stable in the position where you normally sit when you mix....and point the meter toward the phantom center (not the individual monitors). Set the level selector on the meter to 90.
Make sure you have your *monitor* (not DAW) volume control off (down to "0") so no sound comes out.
Now (assuming you have it, if not, download it from somewhere) play back Pink Noise from your DAW.
Just load the audio file on a DAW track, and set the track fader to "0"...set your master DAW fader to "0"....hit Play (you may want to put the track on loop).
With the Pink Noise playing back...slow turn up the monitors while watching the SPL meter. When the needle (or digital readout) is "dancing" around the center or around 85 dB SPL...that is your monitor playback level. Mark that spot on the monitor level control.
If you don't have a separate monitor level control...get one....Mackie Big Knob, TC Level Pilot...etc.
If you want to use some other reference monitor level, then adjust for it.
I have 5 levels marked off on my TC Level Pilot, 80, 82, 85, 87, 90..., with 85 dB SPL as my center point, but I generally monitor at about 80 dB SPL. The 5 markings let me know where to turn the Level Pilot knob so I'm not just guess.
Once you have that sorted out...you know where to set your monitors, so then, the individual tracks and the mix in the DAW are used for setting your Mix levels, not your Monitor levels. If you want to monitor louder...you turn the monitor volume up a notch to a known point, or down if you want to lower it...you don't change the levels of the tracks or the master in the DAW to change how loud you want the overall monitor volume.