Actually, "Total Hardness" is a measure of all of the divalent (doubly charged) cations and in most water this means both calcium and magnesium. The number is usually expressed in units of ppm equivalent of calcium carbonate. Calcium Hardness is typically around 70% of Total Hardness, but this varies depending on the water source.

Magnesium does not precipitate as readily and is not found in plaster so is not relevant when looking at hardness for plaster pools. You should not use a Total Hardness test but should rather use a Calcium Hardness test that specifically tests for calcium only. The test kits recommended on this forum, such as the Taylor K-2006, have a Calcium Hardness test.

Richard