Ed,
I'm not that 101% in this case. Minilab chemicals behave much the same as single-use chemicals. However, it seems to be a better approach to replenish timely than to adjust filtration. In my experience, you will run into crossover filtration problems prior to getting a "too far out color balance". Dmax is usually my indicator, too. If Dmax is not dead black, I'll have to replenish CD.

In my understanding, a control strip is a well-known pre-exposed piece of paper that is analyzed with a densitometer. A test strip is more or less an indicator if something is ok or not. I sometimes use test strips to check if my BX is still ok (which is not that easy to determine as CD)