I need a text field that will yield different values depending on different conditions.

I have posted about this a few weeks ago but I still need the right formula to it.  I need a text field that will yield different values depending on different conditions.  I need a field that will prompt "NA" if a project has been started on time, and them it will prompt L1 if the project is less than one day before the assigned start date and L7 if it's 7 days before the due date.  This is what I have currently