Inside of the forecast tabs you have a lot of functions all based on the total amount from product items on each opportunity. Can I use a custom formula field for forecast instead of an amount field , if so how ?
As far as I'm concerned in the forecast tab you cannot use formula fields. A workaround it is to create an extra custom field that will be populated through a workflow/ trigger with the value from your formula field. For the historical data you'd have to upload a list with the object id in order to trigger the new created workflow.