At a local club I've been recording a stereo SBD feed and a stereo audience pair of mics (four channels) for a while now.
At first I was caught up in the percentage thing too.
But now, after some experience under my belt, I listen to the two stereo sources separately in CuBase. I decide which source I like best.
I use that as the main source. Then I mixing the other source in until I can hear a difference. If it sounds better, I add more of the second source until it starts sounding worse. Then take note of the ratio of the two extremes.
Doing that gives you a good range to work from as far as how much of the second source you can add.
You may have to do things like flip the phase of one recording. Sometimes I even swap the left and right channels on one recording if the stereo image isn't stable.
But, more times than not, I find that the audience recording is the best source and I just add in enough SBD to tighten it up.
Ideally you would continue to experiment until the mixed sound gels for you.
I'm generally too impatient to do much more than that on a typical bar band recording. I make a decision on how much to mix in and listen to each source by itself again, to make sure the mix doesn't sound worse than either source by it self.