I cleaned up a few spots, but the logic still seems odd. If this doesn't work for you can you describe what you want using plain english, and then we can make sure it matches.
If(not Isnull([Override Actual Complete Date]), [Override Actual Complete Date],
If([Total # of Active In-Service Tasks - Job] > 0
AND [Total # of Active In-Service Tasks - Job]=[Total # of Active In-Service Tasks Completed - Job],
[Latest Actual In-Service Complete Date - Job],
If([Total # of Tasks Completed - Job] = [Total # of Active Tasks - Job], [Latest Actual Complete Date - Job], Null)))
