I finally understand this, yay! For weeks now I've been reading about CO2, and it's now just finally dawned on me how to relate this chart to a drop checker. I was so focused on just understanding the process that I didn't even realize you're just checking the pH of the indicator solution. So NOW I know you take that pH reading and apply it to the chart.
Whew! I wish the first post of this thread said, "For use with a standard dKH solution. Compare the color of your drop checker to your pH test kit card, and use that reading (along with your standard dKH) to find your ppm of CO2." ...now that's here in case someone as slow as me reads this.