Apologies for not being better at the maths. I'd always previously (perhaps naively) assumed that it's better to use a local card or cash whenever possible overseas, rather than be hit with the FX exchange fee in order to get AM on a Hong Kong card.

But I'm starting to realise that may be wrong, particularly with some of the better earning HK cards on overseas transactions such as Fubon (and, of course, HSBC overseas promos).

For instance, I'm off to UK in a week where I've got a local card that earns 1AM = £1 (or about HK$12). Also have savings in sterling, which I use to pay off the card, so no FX spread if I use my UK card.

Have always used this UK card in preference to HK cards on previous trips. But given that you can earn 4 times as much with Fubon (1AM = HK$3), even using the standard valuation for an AM, I'm guessing I've been getting it wrong and would be better off eating both the FX fee and spread in order to get the extra miles?