Rule of thumb, a flush is generally 2-3 times container volume from all I've ever read. Never heard it's 2-3 times how much you usually water with??? But why do you need to flush? Just cause you watered with epsom? That doesn't make sense either, and how do you KNOW you had a mag def? Your problem may be nute lockout from pH imbalance and/or salt build up, but that is from fertilizer salt build up, from the chemicals in your hydro nutes, not epsom salt, that may be where you're getting the confusion why you think you need a flush?
AFAIK, you don't need to pH flush water if you use RO water, only if you use tap water, cause RO is 0ppm and will quickly take on the pH of whatever it contacts cause it has no buffering ability being 0ppm. So I would suggest to flush with 6-9 gallons of RO water per container, or if you use tap you need to pH it all to about 6.2 first, and then measure the first run off that starts coming out of the pots and see what it is and how far off it is. You want to keep flusing until runoff is within range.
Lastly, I just want to say that if you have hydro nutes (what nutes are you using?) you should really just give up on soil and run 100% perlite hempy buckets instead. It's just like watering soil, you just water once a day with a decent amount of runoff and it's like a feed and a flush all in one everyday. It's impossible to overwater, and since you're watering so often, the fertilizer salts never have a chance to build up. If you're running organics I can understand running soil, but don't know why you'd think you need a flush cause you watered with epsom??? That is not the kind of salt you need to worry about building up in your soil.