Although you can go to several pH, KH, and CO2 relationship charts...most of which also indicate the formula for CO2, I thought I'd list a couple of others that I figured out so that any one looking to adjust their PH by altering their KH can easily input the numbers. I've punched the formulas into MS Excel and it's much easier to do than messing around with a calculator.
Everyone can find CO2 = 3 * KH * (10^(7-pH))
But, if you wanted to adjust your pH by altering your KH, you could look at the chart or punch the numbers in this way: KH = (CO2/3)/(10^(7-pH))
Just for kicks, if you wanted to find out a hypothetical pH, given CO2, and KH: pH = (LOG(CO2/3/KH)/LOG(7))-7
You can easily make a calculator in MS Excel given the above formulas to calculate this.
Also, if any DIY'er is interested, and uses MS Excel, I have a spreadsheet that calculates tank volume, weight, gallons, and glass thickness with an adjustable safety factor. Just send an e-mail asking for it.
If this is old news....sorry! I didn't see it in a search.