Im brand new to growing hydro... so please don't take what I say into *complete* consideration... but I believe since the ppm should be around 400 or so for seedling lets say... then it would be MOST ideal for it to always be 400.. so if you could create a computer program or something to maintain 400 all the time.. it would be best.

I'm really interested in finding out the answer to this question too... :P