Yeah, I agree that 15gallons is too much, but it's really not too much...
It depends on the toxicity of the soil...
Some people run 25 gallons through a 5 gal for a flush...
Just trying to reaffirm that it's not how much water you put into the soil that overwaters it, it's how often you water...
My method is to saturate with a flushing agent... Wait 2 hours... And rinse the soil with plain water...
I grow in a light mix 50/50 perlite and peat, in 5gal buckets...
I add about a gallon of the flushing mix to saturate the soil, then rinse with 5 gallons... I peak low, and don't have much buildup...
Some people who use organics don't find a flush to be necessary...
But flushing your plants will definately give them a cleaner burn.