I think the pH drop meters are crap. THey are meant to test the PH of clear liquids and hydro nutrient mixes are not always clear, especially GH FloraNova with Floralicous Plus or Floraliscous grow/bloom. LOts of organic supplements are dark in color too. It's hard to get a clear reading when the color of your liquid sample is already yellow or orange or light brown. It's hard to tell which way your pH is going too.
I highly recomend a product that I use.....It is a pH monitor. Cost is $90 US made by American Marine. It uses a probe that remains in your resevoir at all times and gives you a constant reading. It is accurate to the 100th of a degree of pH (7.00). It comes with calibration fluid, but a one-time use amount. I suggest getting a small bottle of calibration fluid to go with it.
My probe is always in the rez, and it is attatched by a long wire to a remote digital readout that is outside the grow space, so I can check up on it lights on or off. It is total peace of mind, and a valuable resource. I like to keep my pH between 5.7-5.9 during veg and 5.9-6.1 during flower. This is what I believe to be the best condition for nutrient uptake for the two different cycles.
Do a search for "American Marine pH monitor" on google or something and you will find a few places that sell them.