Return number from the last filled row of the last worksheet

[DH613]

Hi!
I am trying to reference a number from the last filled row in column "I" on the last worksheet in a different workbook.
The workbook I'm updating contains the expected end of month revenue of business this month.
The workbook I'm referencing contains the expected commissions we will need to pay our sales team.
That number is always contained in the last sheet of that workbook, I add a new sheet every month.
That number is also always contained in the last filled row in column "I" of that sheet, the row number changes based on the active salespeople that month.
So basically, I want to add The last filled row of Column"I" in the last sheet of workbook "commissions" to the outgoing monies in this months workbook.
Any help would be greatly appreciated.

 

[DH613]

This is what I tried, but it doesn't seem to work.
I put a VBA for LastSheetName() in the "commissions" workbook:

Public Function LastSheetName()
Application.Volatile
LastSheetName = ThisWorkbook.Sheets(Sheets.Count).Name
End Function

I then tried to reference that using the INDIRECT formula.
The problem I encountered was that the first code returned different answers than were actually true!!