what I test is nitrates in the tank water, and chloramines in the drip output. I used to drip 40 GPD, and nitrates starting climbing slowly, so I went to 60 GPD, and nitrates don't creep anymore. That's what I'm after, so I've never tested the actual drip rate.
What I test more often than anything is for presence of chlorine in the drip output--I've never seen any. I went 6 months on my last set of filter cartridges, which is better than 3 months when I very first started running the drip. So, I'm stoked.