you are not mathing right.
Plus/Premier package only covers grats for the person with the package, not everyone else in the cabin. The first two people in the cabin must both have Plus/Premier package. Not just one.
So, if there are 2 people in the cabin, Plus is $120 per day: $32/36 for grats, $50 Wifi (unless Platinum/Elite, in which case it's half off) and the rest is for drinks/desserts/casual dining.
If there are 4 people in the cabin, of which only 2 have Plus/Premier, then you must pay separate grats for the second 2.
I know this for a fact because we've just traveled with a Premier package for each adult, while our minor kid cruised without a Package. We paid $112 for his gratuities.