The reason we use a “linear measurement” or “head” in the identification of NPSH and other “heads” around a centrifugal pump is to avoid the confusion that is relevant between liquids of different densities. In essence, two columns of the same height, but of liquids of different densities provide different pressures at the bottom of the column. However, the “head” at the bottom of both columns is the same, because the height is the same.

Your pump supplier has identified that the pump requires 3.0m of NPSH at a specific capacity flow. This can be accepted as 3.0m of any liquid. As you have an NPSH available of 4.8m, you can assume you have a cushion of 1.8m of NPSH.