Automating Monthly Budget Averages Based On Start And End Date

I am trying to create a document that takes our client PO amounts, tactics/spend line items, and dates from one sheet and averages them across the months that it is eligible, so we can know the total amount of money we can expect to go out from our clients each month.

For example:
Client1 Tactic1 StartDate EndDate TotalAmount
Client1 Tactic2 StartDate EndDate TotalAmount
Client1 Tactic3 StartDate EndDate TotalAmount
Client2 Tactic1 StartDate EndDate TotalAmount
Client2 Tactic2 StartDate EndDate TotalAmount

I have mostly figured this out in a really complicated way with many nested if statements, but there HAS to be a simpler way. There will be lots of hands in this document, so I would like to make it as simple and easy as possible.