They make calibration solutions in 4.0 and 7.0 ph with the 4.0 being the most important because you are testing mostly for acid. I keep a supply on hand of both and check my meter at least one time a week or anytime I think there may be some reason to doubt the reading i'm getting. The reason being ph'ing the water is one of the most important aspects of getting good growth. Most tap water that I have seen will test 7.0 or higher and will vary slightly from day to day. The ph of water will even change from house to house on the same street. This makes it very difficult to tell someone how much ph down should be going into their water.
Fox Farms Ocean Forest is a little hot and I would have skipped the Grow Big for the first month or at the most a 1/3 dose once a week. If you think about it the plants you have are really just seedlings not full grown adults so why feed them like adults. When you were born did you start off eating rib eye steaks and baked potatoes or was it more like breast milk?
Since you are just starting out write down everything you do and observe what happens. Learn from your mistakes and each grow will get better. Six months from now you will laughing at some of the things you did. That's the fun part of what we do and the most rewarding.