M to G - tax calculation

Hi All,
Can anybody please remind me how to work out the extra tax to pay when going from M to G with miles? I can't remember how to do this and how it differs if upgrading one way, or both ways etc.. I'm sure it's been mentioned somewhere before.
Cheers
Can anybody please remind me how to work out the extra tax to pay when going from M to G with miles? I can't remember how to do this and how it differs if upgrading one way, or both ways etc.. I'm sure it's been mentioned somewhere before.
Cheers
