You don't need a function to do that, just do

{fieldId1} / {fieldId2}

thank you - that was driving me nuts with such a simple thing. Can I make a formula referencing two other formula fields?

I don't think you can

@MaryPeace FYI the formula {fieldId1} / {fieldId2} will error out if let's say the value of the denominator {fieldId2} is zero so the best formula should be {fieldId1} / nullif({fieldId2},0)