Knowing your current inventory state, forecasted revenue, and planned markups you can figure out how much to spend on each category or supplier. For example, if the current retail value of a category is $100,000 and the revenue forecast is $150,000, you should add $50,000 more inventory to this category ($50,000 Open-to-Buy at retail). If your planned markup is 50%, Open-to-Buy at cost is $25,000.