Best way to pay income tax with Citi PremierMiles card?

Hi everyone, currently I'm using the Citi PremierMiles card and as the income tax period approaches, what is the best way I can earn miles while paying my taxes?

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Understand there are services like CardUp and iPaymy but I would love to hear what's the most worth method those with the same card is using :)

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Also how does one calculate the cost per mile? Would appreciate any tips as I might be calculating it wrongly haha.