How are the monthly forecasts calculated when there are manual adjustments made on customer specific forecasts?

If you have manually adjusted the monthly forecasts for customer specific forecasts, then the overall item (main SKU level) has it's monthly forecasts calculated as follows:

If there are manual forecasts on the overall item then that is used.

Otherwise it is the maximum of either the sum of all the customer forecasts, or the automatically calculated total. 

If this SKU is also a component and has manual demand from the built products, it is the maximum of all 3 values (sum of customer forecasts, manual demand of built products, auto calculated value).