Yeah I have a ppm meter that's good but I've been making it work with the drops(I bought a shit ph tester and I'm not sure that I ever calibrated it right
keep in mind ph should be higher around 6.5 at tis stage in the plants life(coco or soil) and ppm should only be in the 1000 mark range for run off tests. make sure when you are flushing that you get 30% out what you put in, ex flush with 3 ltrs, 1 ltr should be coming out the bottom, this ensures you dont get nute lock and other burning
and when flushing, especially when you have burning present it is a very good idea to use a product that will help pull those old nutes out and clean the roots