RMS stands for "root mean square". Rather than a simple average, it involves finding the average of the squares of the values being measured, then taking the square root of this value. I believe Dolby recommends using this method for Dialnorm because it more accurately reflects the characteristics of human hearing.

I used a program called Wave Editor to find the RMS values when comparing the '93 mix to the mono, though this is only available for Mac. But no doubt there are pc programs that can perform this function as well.

However, it probably isn't necessary to actually go to the trouble of measuring them. Using a Dialnorm setting of -27 for the mono mix and the '85 will give a 4 decibel reduction which should bring them closer to the level of the existing stereo and 70mm mixes dark_jedi already has. It won't be exact, but they will be less obviously loud when switching audio tracks on the disc, which is the goal of using Dialnorm in this case. As long as they're pretty close, it shouldn't be a problem.