Perhaps, consider foliar feeding with your cal-mag, or maybe just with some Epsom salts alone or supplementing the cal-mag. Particularly, consider top-of-leaf foliar feeding using Transport/Optic Foliar. Foliar feeding, even traditional under-leaf type, will resolve deficiencies quicker, and you won't be adding a load of salts/ions to your coco.
Otherwise, has anyone tried to test the limits with MegaCrop? I'm currently feeding 20-30 day old plants at 5 mL/gallon (plus Sweet Candy. Cal+Mag and MEGAzyme at recommended amounts, beneficial microbes, sometimes some silica and/or B-52), plus Transport foliar feeding with a good dose of AN pH Perfect Connoisseur Coco Bloom A&B and cal-mag (CALiMAGic), and I see no indications of any nute burn. Has anyone tried 6 mL/gallon in veg phase?