Mr. Abu Salih

How to determine a spiral volute casing (turbocharger, pump) cross sectional decrease in value? I am trying to design a volute casing. Problem is, I don't know how much its cross sectional area decreases over time. For instance starting cross sectional area (0 degrees) is 100 cm^2, what will it be when it reaches 45, 90 degrees and so on? Is there any way to calculate it? I am sure that knowing starting and finishing cross sectional areas size (btw I know them) and just connecting them isn't a correct way.