No, you can ph after. That is how I do it anyways. I always check ph after adding anything to the water then if it needs adjusting either up or down I adjust accordingly, best way really to insure that your plants are getting the correct ph level.
i have no problem with doing that at all.
just ive looked at debates saying that its impossible to get the correct ph due to the color of the mollases, something like its doesnt read right...guess only way to tell is by doing it myself.
here are also retaken pictures of new leaves affected. still the same problem your thinking(lockout or def)