Heres a link that might help ya out, it breaks down his recipe into smaller portions. As for the promix, yes you can use promix but it is a soiless medium so it has no nutrients, unlike roots organic which is ammended. So you might wanna look into feed with tea's also, it wont be a "water only" recipe.
https://www.rollitup.org/subcools-old-school-organics/553550-supersoil-smaller-scale-grow.html
Heres the recipe for using 3 cubic feet(1/4 batch)
2 large bags of a high-quality organic potting soil with coco fiber andmycorrhizae (i.e., your base soil)
6.25 to 12.5 lbs of organic worm castings
1.25lbs or 20 ounces steamed bone meal
1.25lbs or 20 ounces bloom bat guano
1.25lbs or 20 ounces blood meal
3/4 lbs rock phosphate
3/16 cup or 3 tablespoons Epsom Salts
1/8 cup or 2 tablespoons sweet lime (dolomite)
1/8 cup or 2 tablespoons azomite (trace elements)
1.5 teaspoons powdered humic acid