From my understanding, the credit card is $800 of use, if it isn't paid off then interest will be applied on top of the credit card for every month.
If finance charge just means 'interest' then interest is based off the current debt balance on the credit card. Multiply that and wala! interest charge (finance charge i think)
800 * 0.016 = $12.80
Finance charge is $12.80 for that one month
Or
800 + 12.80 = $812.80 Finance charge (as a total owed in 1 month)
Hope this helps.