Question about ppm , check before and after feed ?

So if I'm checking ppm and I'm looking to hit targets through different growth cycles .

Should I check ppm with pH water only first to get a true reading of the run off ....... Should I be recording ppm of nutes added to the soil then recording the run off a week later when it's time to refeed that way I know how much is being uptaken.

I checked my ppm from last week and ppm is 900.

Just refed and ppm is 2400

So a few questions there about ppm and how do you guage it .

Is this a soil or coco grow? Runoff PPM isn't very accurate, but it can give you an indication if something is wrong, if your plants look healthy there really is no need to check it. If you are feeding mineral salts in a soil grow, I'd feed to plenty of runoff to prevent PPMs from creeping up like that-it works great. Most of us never check runoff PPMs, we just feed on a set schedule and watch the leaves for changes in health. If you are a new grower, aim for perfect leaf health with no nitrogen claw or burned tips, if you do that and are using a complete nutrient, you'll end up with lovely buds.