No issues with magnesium yet but I’ll definitely be on the look out for signs of lockout. I only add 1/4 teaspoon mono potassium phosphate 2 times per week. the strains I grow could be more sensitive to N.I've never had an N tox in flower with Masterblend. I like 3 grams MB, 2 grams cal nit flower week 1-5. Week 6 is 3.6 grams MB, 2 grams cal nit. Week 7+ is 3.6 grams MB, 1.66 cal nit.
If you're adding MKP, I'm surprised you're not locking out magnesium with all that potassium.