I don't flush my systems at all I just have a
pump that drains out about 1/100 of the systems full volume. so in a 50 gallon system I drain about a 1/2 gallon every day that is replaced with fresh water. I have a auto fill on the system so its automatic. I'm pretty good about checking my ppm and recording it in my log book. as you start a new grow in doors you 'll see the ppm every few days increase in consumption. once you flower your plants you'll see a increase as they set blooms or fruit. Since indoor grows turn annuals into perennials you should see after your first fruiting or flowering a consistent ppm usage by the plants. for instance when my room is going full speed i use 250 ppm every day of
nutrients. so i have a bucket i mix up that has a auto fill for fresh water and i pour in my dry floramato and it also has a very small pond pump in it that runs all the time for blending the nutes.
I then have a timer that uses a small pump ( car windshield washer) that turns it on and adds the mixture to the system at what ever interval i choose (every 24 hours is what i do). I just happen to have growtronix system so my timers and relays are easy to set up and change but a cheap Christmas light timer with seconds will work fine.
If you look at my videos on youtube you can see when i built it last year for this one part food.
Lots of ways to do it but my plants are always in terrific condition and i see total flushing as just tossing money down the drain. in a commercial environment i see they don't flush the whole system either they analyses and add just whats needed.
Also in my system since i don't ever need to clean it and i have p[plants now that are several years old in a continues fruit cycle i simply trim there roots like you trim there leaves to control the roots from plugging the system as they like to do.
I have a cantaloupe that will send a root down 10 or 15 feet of hydro farm hose following the current i imagine since it always try's to get to the nute pump tank and it could go in different directions on its way there? just a fyi weird fact
So i just lift the plants up every two weeks and cut the root off about 4 inches or so from the bottom of the bucket straight across like a buzz cut.
Also every once in a while if the roots systems look a little dingy or off white more than normal i add some peroxide but i have no method to this magic its just some thing i have always done and i like the idea it some back to oxygen and water in the system and is non toxic.
As you build your system i recommend you use timers and relays to automate everything. you will be more constant and your crops will grow better as you will now be able to tweak adjustments and see differences. Now that plant food is being commercialized like fast food products be sure to stay as close to the actual nutes as possible the plants need and stay away from the marketing nonsense of growing bigger buds. if that stuff really did a better job commercial growers would be using it. My pineapples and my tomatoes and peppers black berry's cucumbers Lilly dahlias etc etc all grow just perfectly in the same solution.