I like using Belgian malts in American styles. I agree that you'd use whatever you'd use for Crystal 20. I wouldn't expect a lot of crystal malt in an American IPA anyway, so I wouldn't go more than 5% or so. Maybe a half pound in a 5 gallon batch.
Caravienne gives a nice malty sweetness that I like in some styles; it's less sugary to my taste, so I think it works fairly well as a flavor enhancer. If I were trying to boost the malt perception in a pale style, I might use some light Munich and some Caravienne. Maybe 85% 2-row, 10% light Munich, 5% Caravienne.
If I were making it for myself, I'd skip it and just go with the 2-row/Munich mix, but I usually like my IPAs pale and dry.
Why are you adding it? Are you looking for a certain flavor contribution, color, body, residual sweetness, or something else? Every ingredient you use will have an effect, and you should know why you're using it. If I knew your intent, I might be able to give you something more concrete.