Are your plants very big? 300 ppm is lite, if your plants are very big, 5th leaf set for instance and growing decent i'm sure they could handle more. But as always , to little is better than too much.
i dont think theirs enough info to say tho if ur water is 200ppm ya ur prolly to lite. what grow method r u using ? for xample i max out at 800ppm growing aero using.5 conversion ...which is another thing what meter u using?