This is pretty simple to do.
The specifics will depend on where this data is, and where you want to show the percentage complete, but in general, you need two pieces of data - how many cells have any value, and how many of them say "COMPLETE".
For the 'how many cells' question, that's simply:
=COUNTA(D)
this will return the number of non-blank cells in column D (excluding header and footer rows) - in this case, 7.
You can substitute a range rather than the whole column if that fits your model better.
For the number that are 'COMPLETE", that's simply:
=COUNTIF(D,"COMPLETE")
This returns the number of cells in column D (again, excluding header and footer rows) that match "COMPLETE") - in this case, 3
Now you have the numbers you need and it's simple math - number of complete / number of rows, or:
=COUNTIF(D,"COMPLETE") ÷ COUNTA(D)