Say you have 500 ppm with a fresh batch (minus the additives which is good).. which you do.
Keep an eye on the water level.
Next time you check on the plant, check the water level again. It should of course have gone down.
When the level go's down, you also check the PPM (which i'm sure you do).
If its still at 500(or close), that means you are feeding the exact amount you need to be. You've hit the sweet spot for the time being.
If its quite a bit under 500, you would know you need to feed a bit more.
If its quite a bit higher than 500, you need to back off.
With DWC you don't need as high of an EC/PPMs compared to other hydro systems.
500 seems low for a flowering plant, but not for some strains.
600-650 sounds more like it, unless you have a hungry strain. 750 is the highest I would ever go even with hungry plants.
Less is more with DWC.
I would go back to 600, using more base nutes to make up for the additives you took out. Actually.. I wouldn't waste the fresh batch you made though, and use it to determine how hungry the plant actually is the way I described.