Well lets do the math. 35 percent is over ten times stronger than 3 percent. So I would go with 1 tenth of a half cup per gallon. I would go with 1 tablespoon per gallon. Your lucky to have the food grade h202. I can't find it here. If your roots are not slimey with snot maybe less. 15 tablespoons per 15 gallons of 35 percent h202.