Formula needed to calculate this months total with previous data

[pcgs]

Hello,

I work for a professional services company and I am attempting to create a formula-driven invoice system.

Here is what im working with:

Project Phase Previous Year Percent Previous Year Billing % January % February % March % April
$7,500 Phase 1 10% $750 15% $375 40% $1,875 No Percent -$3,000 60% $4,500
$5,000 Phase 2
$12,500 Phase 3
$15,000 Phase 4
$12,500 Phase 5
$17,500 Phase 6
$16,250 Phase 7
Total Fee $86,250.00

They currently invoice on percentage complete. In the cell of the current month, so lets say it's April. They didn't want to invoice in March since that phase had no activity, but it did in April. My current formula is =project fee * (New % Complete - Previous % Complete)

So for Phase 1, January is as follows:
=A3*(E3-C3) OR =7500*(.15-.1)

My current problem is that we don't invoice every month for every project. What formula can I input to relay that information so we don't have a negative value, like March happen, and have April be correct with previous data? I would also like the only data to be input would be that data under the "Percent" column.

[Pepe Le Mokko]

Attach a sample workbook (not a picture or pasted copy). Make sure there is just enough data to demonstrate your need. Include a BEFORE sheet and an AFTER sheet in the workbook if needed to show the process you're trying to complete or automate. Make sure your desired results are shown, mock them up manually if necessary.

Remember to desensitize the data.

Click on GO ADVANCED and then scroll down to Manage Attachments to open the upload window.