^ That will work for ratios anywhere between 0% Mid / 100% Side (with the Mid fader all the way down) up to 50% Mid / 50% Side (Mid fader all the way up).
To achieve ratios between 50/50 and 100% Mid (full mono), you'll need to leave the Mid fader all the way up and lower the other two by the same amount.
If Audacity has a channel linking function, which lets you move both faders in unison, you could use it to do that. With this channel routing scheme, it's not critical to keep the two faders exactly equal (but with other mid/side routing schemes it is). In this case if the fader levels of the two channels are off slightly, the balance will not be perfectly symmetrical and the resulting playback image will be more mid-monophonic on one side and more side-atmospheric on the other. That could actually come in handy to tweak the ambience and stereo-ness between the two sides if off in the original recording. Likewise, if you really want to dive in deep with this kind of asymmetrical correction, you can equalize each side separately. Doing that is similar to Goodcooker's technique of boosting bass in the Side channel, but by a different amount on each side.
Between the ability to adjust levels for all three channels, equalize the Mid, and equalize each side (separately if necessary), you have a huge amount of control. Use it wisely, it can get confusing fast if you go too far afield. What you achieve with all this is a limited ability to tweak the sensitivity, pickup pattern, and angle of the resulting "virtual microphone" on each side separately, and to vary these things by frequency range.