I have heard of 4 as well but im not sure you need both. I always use 7 and mine is always on point. Now that you mention it though I am gonna look into this. I will let you know what I find. Pics of ac box coming right up.
Having both really makes a different (I think, lol). The point is that if you just give it 7, there is only one reference point. Kinda like a sight on a gun with only the part closest to the butt being there. You can put what you think is the center of the barrel in the slot... but having the point at the end of the barrel really increases accuracy. Having two reference point makes things more accurate.
Also... if for some reason your 7 is contaminated or just off for somereason to say 7.5, then using only that your 7 on the meter will actually be 7.5. However if you use the bad 7 with a good 4 or even a bad 4, the spread will still be enough that it gives a much better reference than just one thing.
I feel like that totally doesn't make sense, hopefully you can get what I'm saying.
Love the AC box. That shit is awesome. I admit I haven't read everything in the thread yet, so you might have already said... but I'm guessing the AC box is to keep the room sealed because you use co2? So much more efficient (monetarily) to build a box like that then buy an expensive ass AC with the separate exhause. What are they called, like dual hose or something.
Either way props to you my friend.