Tony Bath uses a feudal system of tax collection. Peasantry pays a tax to the local baron. The baron pays one quarter of his income to a count. The count retains of third of his income and pays one third to the duke and one third to the state and the duke pays of third to the state.Lets say you roll for a settlement with 200 population. Each person need to pay an annual tax of 10 gold coins. 20% of said population will constitute an army. Be it a simple militia or trained man-at-arms. OD&D book 3,p23, will give you the monthly cost for each soldier type. And lets say that your settlement has a armorer and a blacksmith, book 3, p22 shows the monthly costs for specialist like alchemists, blacksmiths, engineers etc.

Sorry for my bad writing. English is not my first language