H.A.F.
a.k.a. Rusty Nails
At this stage of flower that should be great. What I would want to see in 3 days is for the runoff to show a drop.Fed last night. Initial reading was 479. My runoff came in at 1368. I was trying to cut back a bit on the initial.
If it's around 1000 (putting in a hundred or so with calmag) you're good. Feed her in three days.
If it is considerably less than 1000, mix up a batch of food. This is where knowing what the starting ppm is for your nutes (whatever they may be) is a good thing.
If you know that your feed at 25% is about 500ppm (for example) running a gallon through doesn't mean that the runoff will be 500ppm higher. IMHO it means that if your plants were happy and healthy with the 1500pp, runoff, you can safely put 500ppm in without worrying about it - or even more.
Unless you are trying to push the limits and get the maximum weight you can or something, keeping that fluctuation of light feeding but never starving is what I shoot for now. And it's different for every plant for the most part. The Paradise Diesel I have going now is on day 80 of flower. Her sister from the same seed-drop is already dry and a week into curing. In early flower, no matter what I fed her she ate it and wanted more. the other was a "normal" plant.
It's all guesswork, but with the ppm factored in it's educated guesses.