I like my refractometer. I think it is worth getting.
If you are changing 50% of the water, then change 1st before you adjust the chemicals.
Better to adjust before adding livestock.
Salifert MG - maybe ask someone for the english instructions? some reefers might be using the same.
Or borrow a "english" Mg tester to see if your reading is indeed 990.
I think you got everything covered.
Just get some polyfilter, bacteria etc for standby in case of emergencies.