i have only 3 grows under my belt (DWC) with auto's but what i do is set my (500 ppm tds scale) at the start of my grow at 200ppm and as the weeks go by u will see if the plant is taking up nutes by ur ppm dropping slowly thats a good thing i add 50ppm to next bucket change to keep in Rhythm with the growing plant....your aiming for a constant ppm thats the ultimate end result for me...if your ppm is going up the plant is not absorbing the nutes for sum reason and a build up of salts will rise the ppm and cause either nute lock out or tip burn,this is not for everyone but it's just what i have been doing and the grows are just getting better and better.dont love it to death it takes a tender touch its a weed and will grow hardy if it left alone(most times)..goodluck....diffrent strains react to different ppm as well ive noticed..i hit grow diary's and see what others are running(ppm/ec) with that strain