Yea you do need to fush. I usually do it about once every two weeks with flora clean in the water and then drain it again and put fresh nutrients in it again; this prevents nutrient lockout and keeps the roots clean. I use the 3 part series at an equal portion of each thoughout the whole vegetative phase. After two weeks of being in flower I switch to just floranova bloom. That stuff is great, don't even need anything else during flower to grow great buds with that. Even though don't have to I do add sweeteners and other stuff just for the heck of it. I never go above 800 ppms in veg or flower. Also I use h2o2 every time I change the nutrients so it cleans things up a bit. Other than that I just keep it simple with the 3 part nutrients. I find that mine grow quicker that way. All of that Lucas formula shit is unnecessary. Use about 200 ppms of calimagic every time you water and I use 1 drop per gallon of super thrive
Also if you want to know the best way to grow DWC lookup hygrohybrid on YouTube. He's awesome and that's where I learned from