Hi Mike
I just spent 10 minutes typing this and Themuleous's post appeared. I'll leave it as it was.
I have only been keeping a planted tank since April so I may not be best qualified to advise you but I can offer you some words of reassurance as I used to worry about all sorts of things in my early days, unnecessarily as it turns out.
So in answer to your question, how much will your pH drop? It will drop relative to the amount of CO2 you put in. The more CO2 the lower the pH. There is a table somewhere round here that shows pH relative to GH/KH but I can't find it right now.
But unless you are keeping ultra pH sensitive fish, of which there are not many, you do not have to worry about this unless you are trying to breed them, where water parameters have to be spot on.
BPS.
What you now have to do is experiment and see for yourself. I don't mean to sound harsh but there is a perfectly logical explanation for this, so I will do my best to explain.
The problem with bubble counters is that different manufacturers use different diameter pipes to deliver the bubble into the bubble counter, therefore you may have a bubble that's 4mm diameter and I might have one with a 2mm diameter. So if I said to you that you need 3BPS because that's what I'm doing, you would be delivering double the amount of gas that I was, and probably gas your fish in the process.
Therefore it goes without saying that your bubble counter is only a reference for you and you alone. By this I mean that if you were delivering gas at 1 BPS and that wasn't enough then you would have to up it to 2BPS and so on. All you need to know is that you have upped the delivery rate and only you can be the judge of that. Hope that makes sense.
Just for your info, I've got this plastic bubble counter that came free with an inline diffuser so I thought I'd give it a try. Man the bubbles are so small it must be for a nano tank (which mine isn't), so the bubble rate is so fast through it that I've almost got a continuous stream of bubbles. There's no way I can count that fast. But do I need to? I can just about work out if the stream of bubbles is faster when I up the delivery rate but I have to say it's getting ridiculous. I suppose I should treat myself to one with a bigger diameter but it's something you only look at now and again so maybe I won't bother. I've read some posts round here that some people don't use a bubble counter at all.
When I started using CO2 I was using some ugly 'in tank' JBL spiral affair (now resigned to the spares cupboard, now using inline), and I started ultra cautious at 1 BPS and found this was nowhere near enough for my 180l so I had to keep upping the rate until the drop checker went lime green. This is where I'm at today. Most of my plants are pearling nicely, fish are healthy and I ain't got a clue what my pH is inside my tank
Just remember to use 4dkh water in your drop checker and allow at least 2-3 hours for the drop checker to respond to your adjustments. If you already have fish, keep an eye on them too, because they will be gasping for air a lot quicker than you waiting for your drop checker to change colour if you overcook it. No dramas, just lower the bubble rate again.
Hope this is of some use to you.
Chris