Am I getting an accurate CO2 reading?

The pH/kH chart is only accurate if CO2 is the only acid and carbonates is the only buffer in the water, in an established tank this is never the case.

Invest in a drop checker, you can find them cheap on eBay or places like greenleaf aquarium, which isolates an indicator solution so only CO2 affects the reading and will give you a much more accurate reading.

The KH shouldn't drop at all, but if the water, after sitting out over night had only .3 ppm of CO2 left in it, and it had 30 ppm in the tank, the pH would go up when outgassed by 2.0. That's the flaw in the outgassing overnight method for estimating CO2 concentration - we have no way to be sure how much CO2 is in the outgassed sample. The commonly accepted 3 ppm is just someone's guess, based on wishful thinking, as far as I know. My testing showed much less CO2 if you let it "outgas" long enough.

For ease (IMO) you can virtually assume a T5 is going to be twice as effective as T8 due to penetration, efficiency and intensity!!!

216W of T8 would be high light on your tank so you can guess how high it will be with T5.

This is going to mean you will need superb filtration/flow, perfect CO2 and ample dosing.

It would be best to reduce the lighting down to 1WPG for most of the photoperiod and just use the rest for a 'noon burst' if you want to as that amount of light over a large tank is going to be very hard to keep 'on target'

AC

I have one of the CalAqua double bulb drop checkers too. It works fine, but I was eventually disappointed in that I still have trouble judging the colors. Also, I found that cleaning it was a nuisance, and maintaining both solutions was more than I wanted to do. So, I went back to the standard one bulb unit.

Don't expect the drop checker to give you the last word in how much CO2 is in your water. At best, it gets you close to a good concentration of CO2, avoiding the problem so many of us had, where we had 5 ppm and were sure we had 30 ppm, and were afraid to try to increase it. With the drop checker you will at least know that in the area where it sits, you probably have 20-45 ppm when the checker is green to yellow-green. But, then you need to increase the bubble rate a small amount, wait a day while watching the fish every hour or less, to be sure they are not adversely affected, and watching the plants for good pearling. Then, repeat this another day, etc. until the plants pearl good, but the fish don't show distress - meaning they don't all cluster at the surface like they are trying to breathe air, and don't have their colors fading out or are laying on their side on the substrate, swimming oddly, etc. When those things happen you are on your way to killing them with the CO2.

I have one of the original two bulb Cal Aqua units, and mine does not have the reference solution sealed, nor does it have any white plastic anywhere. I asked them if I could add more pH reagent to both solutions to make the colors easier to see, and they agreed that it would be OK to do so. So, I was adding an extra drop of pH reagent to both bulbs.

Both use bromothymol blue as the indicator dye, and, being an organic dye, the color will not remain the same for a long time. It appears to me that after a couple of weeks or less, the color starts to fade a lot. That is why I think replacing the solutions weekly is a good idea.