exHKG it is 10% of full J but ex-outport (TPE or NRT), it is 10% of the I fare.
My kids are young enough that so I've been doing these for a few years now. I usually get the I fare ex-TPE and book my infant ticket with my I fare and then use the redemption for the missus. I've done it ex-NRT and ex-TPE and in both cases, it was 10% of I.
For all of my ex-HKG, it was always quoted to me as 10% of J, at which point, it is cheaper for me to fly ex-TPE and do the extra leg.