heres some info on Lucas Nutrients, Might be helpful.
http://www.rollitup.org/hydroponics-aeroponics/4387-lucas-formula.html
https://www.gardenscure.com/420/hydr...s-formula.html
International Cannagraphic Magazine Forums - Calculating Nutrient profiles for Canna, GH and PBP

If you change your solution every 7-10 days I wouldn't worry about PPM
to much. You know that your solution is the right mix.
You know that your PPM is gonna go down over time.
You know that when you add pH Down your PPM is gonna go up some.
use distilled water on your next nutrient change, and see what happens.