<If the nutes are in the soil,, watering to runoff would depleat the nutes in the soil in a hurry, but they wouldn't last the entire grow anyway. At some point you will have to start adding nutes to the soil especially during flowering. As far as i know, there are no soil mixes that provide all the nutes nessary during vegging and flowering>
Well said Gordo. About the 20% runoff. I never do that. I water all around the pot. As soon as I see water coming out of the bottom, I stop watering. I also alternate water and nutes.