Get out of that mindset of watering a given amount every so many days. Water that sucker until you get run-off. Then feel the weight of the pot. Let the pot dry out before you water again, this time feel the weight of the empty pot. That's how you judge when to water. Give them what they want, when they want it, and always water until you get run-off. Keep the soil from separating from the sides of the pot. If it gets too dry, the water will run right out. If that happens, place the pot a the tub full of water and let it soak.
As for the soil, you're always better off with soil that has not been "pre-nuted". Start out with a soilless potting mix, then add compost and any other amendments you plan to use. If you do use pre nuted soil, remember that the nutes in it will get you thru 90 days or so, well into most flowering cycles.