Meter the output of your stereo mix but set the meter so you are seeing the
rms level of your mix. It wiill average around some point down on the scale. Note that reading.
Drag your ref track onto a new track and set its channel fader so you get the same
rms reading on the stereo mix. (Make sure you are only hearing the ref track at this point ie solo it)
That should get you in the ball park. The ref track is bound to be louder than your mix as it was probably mastered. So you will find the channel fader for the ref track will be lower than unity in order to get it to match your rms mix levels.
That is one way to do it if you want to use the standard metering provided. If you are prepared to invest in a VU meter plugin there is a another way to do it. It will be a little more accurate. I am assuming you want the ref track level to match your main mix.