either a running total, which is not my forte, or  variable, and you can put the logic in the group footer to increment it as needed...at least that is what I would do.  Since I'll be honest, I don't quite understand the logic of being late, but it seems if any part is late, then it all late, I would do a sum of the late field in the group footer and if it greater than 0, I would increment the variable counter.  Display the variable as appropriate and don't forget to reset it to 0 in the appropriate group header (if needed)