If you're invoicing your Actuals / Bookings you can filter for all bookings in a period, e.g. Jan - March 2020 and then report on the Projects Final/Invoice view, to also include eventual Project Extras that were added.
The "Date" farmerswife looks at when you use the Financial Lines In Range is either the Booking Date of an Object or the date a Project Object/Extra was added or the Date a Budget Line was added to a Budget... so this needs to be considered when gathering the input for the Financial Report.
The below example:
Filter in Advanced Search:
Select all Bookings (make sure no Max Hits When Searching apply!) and run Financial Report On Selected Projects.
In the Financial Report window, select the date range (again).
Choose / import the "Monthly Forecast" template. Ready for download at bottom of this article.
