A full flush is 3X the pot volume with just tap water if that's all you have. If it's really shitty like 400ppm and pH 8 like mine then go buy some RO water forever so you don't screw up your next grow. Don't need to pH that but once that's done pour another part volume, more than needed to water the plant when dry but not a lot more, that is pH tested with 1/4 dose of normal nutes. That will push out the flush water and reset the root zone. Otherwise the plant is sitting there with nothing to eat until you can let it dry out so you can feed it something.
A full flush is the last resort. Things have gone FUBAR and it's your last attempt to save something you think is worth saving. Is it worth the effort? Might be easier to get some cuttings and let the momma go. Do better with her kids.
It takes forever to do too. 5gal pot x 3 is 2 hours of pouring water thru then getting rid of the runoff. Shop vac is handy for that but still have to dump that out a few times as well. If you can move the pots to a bathtub or a big rack with drainage then it's easier but still a PITA.
I use soilless ProMix HP with RO water and AN pH Perfect nutes and never have runoff or pH problems. Just the basic 3-part with Big Bud for my booster. Any old calmag will do and Epsom Salts are cheap like borscht! Works great for my DWC projects too.
Tip: A rolled up towel is great for getting runoff out of the catch tray. Just park an empty tray beside it and wring out the towel into it then put the towel back to suck up more if it's there. Then hang the towel up to dry in the grow room if you have low RH or take it out to dry elsewhere. I wash mine when it gets grubby but that's not often. Great for mopping up spills too.