Before you add any nutrients, what kind of promix is it? I know it's not flowering plant mix because that would've caused a nitrogen toxicity. If it's hp I'd do what bernie says. If it's herb & vegetable, be careful, it has enough nutrients to keep your plant going for a month of active growth. The mycorrhizae in the promix will have colonized the roots of your plant by now so root rot is less likely to be an issue.