Advanced nutrients says they recommend 20% run off when using Flawless Finish. You use it the very last week.
They have since proven that flushing doesnโt really do anything. Dr Bruce Bugbee put all the myths to rest in his labs and said pretty much do not change anything the entire time. He did say that there was no change when using no nutrients the last week vs using nutrients.
I just switch to straight pH balanced water and water enough to get roughly 20% run off.
I hope this helps.