I wouldn't suggest using the cap to measure you pH up and down. That stuff will burn your skin if it gets on you. I pour mine into a cup, then use a syring to measure how much I am using.
For my nute solution, I mix 9 gallons at a time, and I have to use about 90ml of pH up to get my pH from 5.2 up to like 6.5. Be careful how much you use. With the veg formula of FF nutes, I never had to pH. But with flower you will.
You will most likely need more of the pH "up" than you do the "down". I only use the pH down when I am flushing. The nutes will bring your pH down for you, but when flushing with plain water, tap water usually has a high pH so you need the pH "down" solution.
Just be sure you get the pH meter, and not those strips. LIke I said, with the FF nutes, once the Big Bloom is added, the water looks like mud. You wont be able to tell what color the solution is. Even with the GH nutes its not too accurate because of the redish color of the nutes. Also, make sure you get a ppm meter. This way you know how strong your nutes are, what ppm they burned your plants, how stong to mix things for seedlings, clones, mature plants, etc. These two items will increase your yeild and potency a ton. You can give all the nutes you want, the most expensive brands, but if its at a pH of 4 and nothing can be absorbed, there is no good coming from it.