According to the Salesforce Manufacturing Cloud documentation, you can use the Formula Builder on the Account Forecasting page in Setup to create formulas for forecast calculations. You can define your own formulas for quantity and revenue based on sales agreements, orders, opportunities, and account metrics. You can create either a single formula for all periods, or multiple formulas for different period ranges. If the forecast display period is 12 months, you can create up to 12 formulas. However, if you want to calculate an account based forecast for the next 13 months, you need to create separate formulas for periods 1-12 and period 13, because the Formula Builder does not support more than 12 periods in a single formula. This way, you can ensure that the forecast calculations are accurate and consistent for each month. References: Build Formulas to Calculate Forecast, Configure Forecast Metrics and Formulas