Although TNC is one of the better AIO ferts for value for money, if you after increasing your Mg and Fe going down the double triple dosing route TNC soon becomes costly for the few extra nutrients plus it will not address the Fe EDDHA either.
For the Mg to hit 5ppm a week for 200l
So just dry dose two teaspoons of Epsom salts (MgSO4.7H2O) after WC and that will yield the extra 5ppm Mg
For the Fe EDDHA I would keep it simple, buy 10-50grams, add it to 1litre of water and just dose your tank a few drops every day, when the water turns the slightest pink you have added enough as the the Fe EDDHA will be in excess. Your first dose you might wont to add it in 1.0ml doses till you see the slightest change in colour, if your plants use up the Fe EDDHA the colour change will go. I use it with the slightest colour change of water, which is best done looking down the length of the tank.