I top off and adjust ppm and ph whenever the res is down 1 gallon. Don't sweat the minor swings of ppm you are experiencing. Usually, i change water once a week. With all the chemicals you are adding to the nutrient solution, you do not know which ones the plant is consuming. and what chemicals are being left behind and you are only measuring total dissolved chemicals. Without a water change and continual topping and adjusting total ppms you might end up with a nutrient solution that is weak in the chemicals the plant is using and strong in the chemicals is not. I have heard that you should change nute solution when you have replaced the resevoir capacity. In other words. if it takes 5 gallons to fill the res when you have replaced 5 gallons it is time for a water change.
I do not mean to get in an argument with hotbunz, just expressing a different opinion. As long as it all works it is all good.
Your plant is looking very good by the way. Keep up the good work and get ready for some explosive growth.