You really should never put Miracle Gro on MJ; it's not meant for it. As far as the math, yeah I think you're right tho. Stick with the Humboldt. Also, every 5-7 days seems to be too frequent. Just make sure to keep an eye on your plants and be ready to flush them good if they start getting nute burn. I would venture to say that more plants are lovingly killed by overnuting than any other reason.