If condition to calculate the tentative completion date for a project

I need an if condition to calculate the project tentative completion date ,

the givens as a base for calculation is

1)- sequential set of steps for a project .

2)- % of completion for each step .

3)- Overall % of Completion as of today .

4)- and the remaining not started yet steps .

What i need is an equation to estimate when this project will end base on the % of completed steps so far and the remaining uncompleted steps .

Thanks in advance for your support ,