When you have filled your system to the amount you want, record everything. I will use 500ppm for a figure. Now, tomorrow, when you top up your system to the same level with water, take a measurement and record everything again. This will give you the true reading of what your plants are doing. Once topped up, if you get a reading of 460ppm, it will then tell you your plants uptake was ~40ppm! Not rocket science by any means, and letting the ppm's and PH drift a bit is a good thing if you ask me.