Tax:
Base tax = Slaves/2
This gets modified by a lot of things, including culture/religion, land connection to capital, stability, governor corruption and finesse, country/province modifiers, tradegoods, etc. Have a look at the tooltip to see exactly what's affecting the province's tax income.
Manpower:
Manpower = Freedmen*1.25
This gets modified by culture/religion, tradegoods (fish) and governor corruption. There are a few country modifiers that add manpower bonuses, but they are bugged and don't work.