I start around 300ppms and move up by about 100ppms each week until they start showing signs of toxicity. Then I draw it back and will try to push it a little higher during different stages. In short, I let the plant tell me what she wants. If my pH is dropping, I know I've overfed, if it's going up I know I'm under feeding. If it stays relatively static, I know she's taking exactly what she needs.