Chuck's target CO2's are a bit low, bump everything up one section to the higher side.

That assumes that the KH is 100% bicarbonate, alkalinity can be influenced by a number of things, careful with that assumption

I've never seen a assumption that under estimates CO2 yet, there might be one, but I've never ran across it yet after decades.

So most assumptions tend to suggest that you have more CO2 than you really do...........so folks end up with algae due to low CO2.......thinking that are certain they have enough.

But it's still based on that original assumption.

KH ref solution gets around that, but the drop checker introduces others, slow response time, issues for some with using a precise pH color or measured value for pH, I use a pH probe and encase it in special membrane and KH ref solution to get around that.

Regards,
Tom Barr