My system i net pots in 5 gallon buckets with drip feeders runnning 24 hours a day. my system is fully automated so when i need to dump or flush my
nutrients out i have a
pump that is just for that. My growtronix turns that pump on every day for 30 seconds and dumps some of my resevior out. then the system is connected to a water supply so it autmaticlly refills what was dumped out with fresh water. then i come allong and add
nutrient manually once a week or depending on whats fruiting at the time.
if you put in watercatwn6535nd in the search feild at you tube i have posted a few videos of my grow room that might hep you visualize the system.
My point was i never get rid of all the fluid in the resevoir and i have very healthy plants and my nute bill is probbaly a quarter of most of yours. a lot of product is dump donw the drain.
i would challenge every one to run there system tell they actaully see the plant lock out a nutrient. thats almost impossible unless you have really bad water to start with. but then you can have idea of how long your plants take to eat whats in theer and you can tell by inspecting the plant what it is defiecient on so you can then take your ppm reading and know that its probably just the nutreints your plant is currently not in need of.
this is more difficult to explain than i may be able to get across. lets just say the longer you grow the better you know your plants and the cheaper it is to raise them.