How to add a value to a cell when another cell contains a text?

[IamTJ]

Looking at my attached example,

When Step 1 is completed, I will enter the date of completion.
When I do that, it will add the stated percentage (2% in this case) into the Percentage Completion cell.

When I enter the date of completion for Step 5, it will add 20% to the Percentage Completion cell, resulting the cell to show "22%".

Is this possible?

I've tried playing around with "=IF(ISBLANK(C5),"",5)" but to no avail.

(P.S. Steps might not be completed in order)

Help is much appreciated!

Thank you!

TJ