The computation of trade taxes is fairly simple. Each province generates a "base" level of trade tax (called here the BTT), based on the size of the trading post or settlement. This number is then multiplied by the owning country's trade efficiency percentage, plus one. That is,
trade_tax = base_trade_tax*(trading_efficiency + .01)
The base trade tax for trading posts and colonies is their level, in ducats. That is, a level 5 trading post has 5 ducats for its base trade tax. A level 8 colony (800-899 population) has 8 ducats of base trade tax. For cities, the base trade tax is a logarithmic function of population, according to this table:
Population BTT 1000-5000 10 5001-10000 11 10001-20000 12 20001-40000 13 40001-80000 14 80001-200000 15 200001+ 16
As is the case for other forms of province income, when an enemy army has covered or controlled a province, it generates no trade taxes.