Plants are funny are they not? I try not to recommend anything over what GLN recommends for feeding. Let's say you approach it as a mag def and you're already at 6.5g/gal. The tendency is to add more MC. By doing this you add more NPK all around.
We know that macro and micro nutrients rely on each other for a healthy plant. Too much of one can prevent another's availability, or require more of another. So you have a mag def and you add more N and P because you increased the amount of MC. Both of those elements affect Mg, which means the plant increases the demand for more Mg. It's a vicious circle that needs to be broken.
On the other side, too much P reduces the availabilty of Ca and vice versa. Too much P also reduces the availability of K.
MC is designed, imho, with an imperfect NPK ratio and eventually you're adding too much of other nutrients causing problems.
Growers need to understand this and have the tools to adjust accordingly by understanding how certain elements affect the others.
That's my .02