I have had lots of joy doing one day of cal mag at 300ppm, which is the 1ml per litre plant magic cal mag.
Then the next day I just feed normal nutes alone at 600ppm. Then swap back each day.
Then last 2 weeks I go with advanced nutrients overdrive at 300ppm, which is 2ml per 1 litre.
This means every 48 hours I am charging the coco. It also means I don't overfeed. Plus I don't get nutes build up. The plants love being fed this way. It's like the cal mag feed the day before helps the plants east yesterdays feeds.
If you let it dry out the coco ph will go tits up... or tits down
Or just use canna coco a and b every feed, It's got plenty of extra cal mag in.
Why don't you flush the coco in the shower, then charge it with ph'd cal mag and go from there?