I mean, I can get 10 gallons of RO water....then add some of my water back to the mix, but to understand what the resulting chemistry of that mix will be is highly challenging to my understanding of what I have actually arrived at
When combining two different waters, just do a weighted average for each ion. (That isn't technically correct, but it's certainly close enough for our purposes.) If you're diluting with distilled/RO water it's really easy: just multiply each concentration by the fraction that's tap water.
So, for example, if your HCO3 is 337 and you mix the water 50-50 with RO, the resulting water will be 337*.5 ~ 168 ppm HCO3. If you use 2 parts RO, 1 part tap, it'll be 337*.333 ~ 112, etc.
Your biggest problem will probably be sulfate. For anything other than a very hoppy beer, you'll need to dilute at least 3:1 just to get that under control.
This may help, depending on where the difficulty lies: http://seanterrill.com/2009/08/08/water-water-everywhere/
It's a decent starting point, at least.