Additional Scenario in formula to calculate value of column at End of Month

Dear All,

I have had help in applying a formula to calculate the value of the total amount (sheet 2 column A) per month after the exit date is taken into consideration per month.

Please find attached for the formula I was looking for in Cell B2 dragged across in sheet 1.

However, my problem is this amounts starts with a static amount from cell A2 sheet 1.

What if an amount was added in September 18 (350,000 for example), how would I automate this?
Therefore the amount in cell F2 sheet 1 would increase by this amount, and not the months for May 18 - Aug 18.

Would it be easier if there was another column in sheet 2 with a entrance month?

I have reattached the spreadsheet.
Your help would be much appreciated.

Regards

N