I'm putting about 15 gallons of fertilized water a day into the room, just watering the plants that are dry or mostly dry. I watered tonight, reverse osmosis water only, from a 4 stage counter top unit by freshdrinkingwater.com. I replace all the filters every 6 months, since my well water has a really high ppm.
I start with the Cal/Mag, wanting the ppm of the solution to be about 350 or so. My starting water ppm is usually in the 30-50 ppm range. Then I add the 1-4-5 bloom fertilizer to raise it to the level i want- normally between 610-740 ppm depending on how the plants are looking.
Meter wise, for years I've been using the Combo Meter, by Blue Lab. I think they are calling it something else now, but its a continuous meter meant to stay in the water at all times. It measures ppm, pH and temps when turned on. Every drop of water that hits the plants goes in at 6.3pH, no matter whats in it or not. Even fresh water is pH'd to 6.3.
For calibration, General Hydroponics sells a liquid kit to adjust the probes pH. It readily available on Amazon, as is just about everything else I use. My daughter has a high level management position with Amazon, traveling the world. So I use her prime for quick deliveries and discounts.
I constantly stir with resevoir, which adds oxygen to the water. I also wait about 5-10 minutes for the meter to settle on to a number, as it will fluxuate for 5+ minutes. Don't water until the ppm number locks in.