Hmm..., do you mean you car capacity is in Gvkm/a? But surely the investment costs must still be based on car prices, no? If so, I guess you must have been converting them from, say USD/car to MUSD/(Gvkm/a)? In that case, you know what those unit conversion factors are, and you would just need to take them into account when applying NCAP_FTAX / NCAP_FSUB or NCAP_ITAX / NCAP_ISUB.
I am not sure what you mean by an investment tax-credit being proportion-based, and so find it hard to comment on that.
I am not sure what you mean by an investment tax-credit being proportion-based, and so find it hard to comment on that.