How to change formula based on selection from Ref column?

[Sam_Key]

I am trying to create an expression that makes a calculation based on the selection of a ref column.

[Ratio] this is the Ref column with 3 options
[Rise] user enters number

I have the following columns based on 1 of the options:

[Inches] Formula: ([Rise] * 12)
[Feet] Formula: ([Inches] / 12)

I need the formulas to change if a different [Ratio] is selected.

[Steve]

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