Hi Mike
JamesM said:
If the fish can tolerate it, there's no problem with a yellow or near yellow DC.
I'll agree with that, mine's virtually yellow all the time now during lights on after finally sorting out my CO2 delivery. Of course the plus side of this for the fish is, my tank looks like a jacuzzi just before lights out so plenty of O2 for the fish

.
You'll end up just using the drop checker as a rough guide, and let's be fair, your looking at a moment in time some 2-3 hours ago anyway

but it's the best we've got for now.
Like yourself I was ultra cautious at the beginning with the injection rates (boy did I pay for that with the dreaded BBA, but that's all sorted now

). I was quite surprised at just how much my injection rate has gone up since I started, although the extra plant mass has got a lot to do with that.
As JamesM says, watch your livestock, it's the quickest guide you'll ever have.
With regards to your opening post, I have found that when the high pressure side starts to drop, it starts dropping very quickly (within a day on my 180l), and I then get erratic bubble rates, so I change it then. Fortunately I've never had this mysterious 'CO2 dump' that I've read so much about

.
Good luck.
Chris