OK, now i understand.
I suggest that the key field of the Project forecast needs to be changed to a field called
[Related Project - YYYY-MM(key)]
It will get populated by a Form Rule which changes that field to the value in the field
[Related Project - YYYY-MM (formula)]
The field for that would be
List("-", ToText(Related Project]), [YYYY], [MM])
ie the user would create a new forecast and indicate the YYYY and the MM in drop down fields. The formula would calculate what the key field would need to be and the form rule would force it into a text field, since the key field to a table may not be a formula field.
On the revenue records which are your Task records, you can calculate that same Key field. You know the related project, and you can calculate the YYYY-MM based on the [Task-Billable Date].
Then you use that calculated field in the relationship to project forecasts as the child records are allowed ot have a formula field to calculate their Parent.
