Wherever your DAW shows a ratio of your mix, or any numbers relating to that, cut out a small piece of a Post-It note and cover that up on your monitor, never look at it again SBD mixes vary so greatly in my experience that formulas are useless at best, or dangerous at worst (because they discourage critical listening).
FTW!
Who cares what the ratio is? It's never going to be the same twice. Unless the two sources are level adjusted beforehand so that they have identical RMS values before combining them, a ratio of the combination of the two is completely meaningless. And even if that was actually done, identical mix ratios between two different recording dates would still produce different subjective results, because differences in content, spectrum, and dynamics between the two sources will require different mix ratios. Even if RMS level matched, mixing the two source based on a ratio is meaningless.
Mix ratio only has meaning in some audio software as a rather clumsy way of adjusting the relative levels, and even then only within
that particular session with those particular files. Whatever ratio works optionally for one recording usually will not translate to the next one.
All it takes is listening with a hand on the level controls. Figure out what relative combination levels sounds best, then adjust the level of the resulting combination.
Just like the admiral confirmed as well.