Rule of thumb for flushing is at least 10 times the pot size. You wanna wash out ALL the salts. After you've done that, fine. Now, do you have access to any cal mag? Magnesium and calcium are dependent on each other for uptake. If you can at all get cal mag then do so, otherwise yes, get some epsom salts and water one last time with anywhere from 1/2 to 1 tbsp per gal as your last "flush". That will give it some Mg. can you check the pH of the runoff water? It should be at 6.8 or so. Too high or low pH will affect uptake/availability of nutes. I doubt you have a pH problem, but then, better safe than sorry. If you don't have one, invest in a good pH pen.