Start from the beginning then dont just wildly jump in!
What is the ppm of the water you are feeding them without ferts? My tap water needs adjusting up to 150ppm with mag as it is below 50ppm out of the tap! Is it the ferts or the tap water that are lacking in the mag? I dont really get cal/mag problems since i add the right amount of lime to my soil to buffer pH and magnesium/epsom salts to my water to get its correct ppm.
Test your water first to see what its ppm is, alot of the waters ppm is cal/mag so if it is low then you need to raise it simple. If your soil is acidic you need to add lime simple too! Dont make it harder than it needs too be, check the water and go from there. Reading runoff wont help you except to know to add more lime and that you might have added to many ferts.
For a one off cure for mag deficiency add 300ppm of epson salts to the water. Maybe repeat again if needed but this should cure most deficiencies by now. If this works keep your water ppm to between 150 and 200 by adding cal/mag. Magnesium also buffers pH as it is an alkaline ion. Get the cal/mag right and pH will be a lot less problematic.
Dare i recomend a thread for you to read, i have recomended it so many times but dont know if people read it, it is a sticky and first three pages teach you about water, cal/mag, RO water, distilled water, pH etc! Well it is a good start to a complicated and complex subject, anyway here it is -
https://www.rollitup.org/general-marijuana-growing/294579-water-most-essential-compound.html
Good luck.