Well in truth the PPM is only good for the time you make up the nutrients formula. The plants take what they need and deposit back-unused portions of your nutrient mix. That by its self changes the PPM number and you no longer have a clue of what the NPK ratios are any longer. If you use enhancers then the PPM number is only good for your NPK mix anyway.

Most experienced growers will change the water every 5 to 10 day's depending on how much you started with in the first place. It very safe just to return the volume of water in your rez back to normal operating level with Ph adjusted water and not over feed the plants or risk burning them from to high of a nutrient level.

That is why most add just Ph adjusted water in-between rez changes.

Hope that answered your question.