You thing you can do is simply add the MgSO4 to the Trace mixture.
TMG has some Mg in it.
But generally, not enough.
Mg defiencies are a lot more common as many assume their GH is a nice mix of Ca and Mg.
Often it's not.
Adding a tad more Mg will/should not harm anything I know of(say 8 ppm per week).
That should more than supply a non limiting amount to plants.
Few planted tanks will ever use 30ppm of N per week.
So Mg is about 7-8x less so the min amount for non limiting levels are likely around 4-5ppm per week for Mg.
I resolved this issue by adding GH booster which has K+, Ca, Mg and if you add enough, say 2 extra GH degrees, it'll resolve most all issues relating to Ca, K, Mg.
If you know more specifics about the source tap and it is consistent(often assumed that it is, but rarely so), or use RO/DI etc, then you can tailor more.
Perhaps all you need for James' tank is some MgSO4.
In that case, just add a few teaspoons of Epsom to the TMG jug/CMS +B etc.
Regards,
Tom Barr