To be honest as long as i have been growing i only found out the other day that i have never actually flushed my soil properly.
This guy on a different forum made a faq type post about leaching.
I had previously thought yeah 3 litres flushed that'll do

I always have leaf curl slight nute probs as well.
He said he uses 4 gallons to flush each pot and he uses a ph/ppm meter to test the water before he uses it so he knows what the original water ppm is then he keeps flushing through with about 4 gallons in total until the ppm was hardly any higher than the water he started with.
He reckons that this then clears out nitrogen deposits salt build ups etc. lock outs the works and the plants can then start to really use all the nutes properly again and that a lot of nute burn assumptions are actually n lockout.
To be honest hole i been growing a long time but i am technically crap and theres a lot i still don't know
Anyway im going to leach my soil properly under this advice this time around in my own grow and see if i notice any difference.
Hope this is of use to you