All that is academic. No Nitrogen test kit at the Hobby grade level can possibly return consistently valid and accurate readings.
The test kit readings are not acceptable, no matter whether they confirm one's suspicion or not. You would be well advised to stop testing for Nitrogen compounds. It wastes time and energy better well spent on achieving good flow and distribution
I could do with a little explanation of something that I could be overthinking ref flow/distribution. So the idea of injecting co2 properly is that we dissolve the bubbles in the water right, as best we can, so I'm right in thinking that actually seeing bubbles all around the tank isn't the be all and end all?
Seeing bubbles may or may not indicate whether there is good dissolution. There are lots of combination of things that can add up to goodness or badness. So, it's entirely possible to have bubbles everywhere and to concurrently have great CO2 dissolution, and it is also possible to have bubbles everywhere and to concurrently have lousy CO2 dissolution.
The bubbles help to show whether it's likely that you have good distribution.
CO2 dissolution is a completely different factor from distribution. Both need to be good, but the problems associated with them are attacked separately.
I agree with your conclusion that if the pH profile is good and the plants sway in the breeze then this indicates that both factors are addressed properly. Unfortunately, this has nothing to do with the DC because the DC has problems of it's own due to the physics of gas movement in a liquid
and the concentration of the water's CO2 is not even close to being the same at every physical point in the tank. The CO2 concentration that the DC sees is different than at a location just in front of a leaf. So no matter where you place the DC, it would be a coincidence if it matched what any plant is seeing.
Sooooo, the way to interpret a DC is to first examine the plants and the rest of the tank with your eyes. When things are going well, note the color of the DC, and when things are not going so well, also note the color. Understand the patterns of color changes throughout the day and compare it to the pH profile that you measured. In that way you calibrate the DC mounted in that particular position. You are free to move it around to try and find an "ideal" location, but this is pure fantasy. No matter where you put it, you still need to correlate it's readings to your observation of the tanks health and the pH profile. Many people report a lovely green DC and the tank suffers terribly from CO2 starvation.
Know this: CO2 is notoriously difficult to measure accurately, and that is true of trying to measure any gas in any liquid. That's one of the reasons Nitrogen test kits are not accurate.
We have a dropchecker guide in the Tutorial section of the forum, even though we are aware that it's basically a caveman tool, but any real tool will cost a lot more money than you'll ever want to spend.
Cheers,