I always started my higher phosphorus regime a few days before flipping to 12/12 so the plants were ready with everything they need to bloom as soon as the clock started ticking..
And there isn't need to drop down to weaker dosages.. The root system should be strong, and the plant thriving and ready to eat up a normal diet..
Don't get me wrong, I've never used that product, and I don't know what kind of concentrations they suggest.. I use EC to determine the nutrient potency the particular plants in question like best once they're growing strong, and use that as a basis.. Point is, in my experience plants are quite happy receiving their first dose of high P nutrients at roughly the same EC they received their last dose of high N food..