Ya you would want to keep slowly lowering the feed like your doing. 50 PPM increments is good. I'm guessing if you added 400PPM solution, it will be around 450 or so, and closer to the sweet spot.
You are gonna find that you need to change the res at least 2 times a day though to keep it that way. Or top it up with plain PH'ed water halfway through the day.
You might need to go even lower than 400ppm with your current setup. Don't think of it as its only getting 350, because at the end of the day it would be getting more until you hit the exact amount. You might find that using 300 is the spot, and at the end of the day you have 300. In that case, you would just top it back up with 300 again, and not even have to do a full change but once a week.
Your at week 6 anyway and GSC can't be more than a 10 week strain. At 6 weeks the plants will noticeably start eating less. I've only grown a cross of GSCs so can't be too specific. I know you could ride out the last 4 weeks or so with low PPMs though. I prefer to just run a low PPM towards the end anyway, and let the plant consume itself. Some flush with plain water for 2 weeks or more. I don't really flush at all anymore. No need to if your not overfeeding.
How many gallons a day is she drinking would you say? 1/2 gallon or more?
Is it getting low enough that the roots aren't being fully submerged? Roots drying out isn't a good thing. You need to keep the water level the same at all times. A little lower isn't bad, but more than a few inches isn't good, especially in a small container. If the roots are halfway submerged, then the plant can't really use the half that isn't. The bubbles popping at the surface do have somewhat of an aeroponic effect in the way that they mist the roots coming from the net pot, but only so high.
If you had the plant in a small net cup, like a 2-3 incher, you would be able to fill the bucket with more than 1.5 gallons. I'm guessing you are in a bigger one though, like 6 inch or something.
I think you need at least 5 full gallons of water in your container. As in, replace the bucket with a bigger container\tote, or add an external res if you can't fit one. Or a float valve hooked to the RO (or a higher bucket using gravity).
I would opt for the $10 or cheaper plastic tote myself. Then everything else I've said would be more important.
Your gonna have to deal with wild fluctuations and multiple res changes to keep the solution stabilized if you don't.
Plants actually excrete waste back into the solution through the roots. Especially during flower. Think about it that way. Like having a big gold fish in a small cup of water. Until you give him a big enough tank, he will start to suffer no matter what.