Project completion focus (best case)

You question isn't clear. There are many, many factors to take into account to schedule construction.

If you have to build 670 stores starting in January and finish them by July, and it takes 60 days to build one, you need to start building the last store by May 31 to finish by July 31. So if you start January 1 you need to start construction on 670/5=134 stores per month from January through May.

That's a gross oversimplification and I didn't need Excel to do it. If you need a more complex model, you need to describe what are the variables and how they affect the rate of construction (availability of labor, equipment, materials, permits, etc., etc.), any other reasons why you wouldn't start the same number of stores each month, etc.