My tank is 20 gallons with a sump, around 25 gallons total volume. I use instant ocean sea salt (not reef crystals) and keep my magnesium level around 1400. I dose about 100ml whenever my magnesium gets around 1350-1320 (which ends up being ever one to three weeks).
The product I use is "general adjustment magnesium mix" from BRS. This is a powdered mix of magnesium sulfate and magnesium chloride that you mix with 1 gallon of RO/DI (you dose the liquid, not the powder).
So far I've used around half a gallon in like a year.
Reef Advantage Calcium also has 17mg of magnesium. If you have a coralline algae and lots of it boosting maybe needed if you have LPS dominant tank You don’t have to test magnesium as frequently as you would test calcium/alk Dosing by hand could be more of problem but if you have dosing pumps...