Transplanting in rock wool

I’d probably top feed for a bit personally- I use rockwool quite a bit but never used the 6” . I usually just use for propagation. If you’re gonna do the flood half way up the 6”- I’d think that would be totally enough to wick up the entire rockwool cube but thinlizy has a good point about flushing salts.

Looks like you cut into the 6". You didn't need to do that. All you have to do is sit the 3" on the top. You can flood and drain a 6" cube if you stack them you should feed from the top. You might get away with the flood and drain. Your roots will take and will be good for the full grow. If it was me i would feed from the top drain to waste. That way you are always flushing excess salts.