Assuming that I understood your problem correctly, I hacked together a quick solution. First, you should ensure that you create only one outline entry per day, if you are checking in and out multiple times. Define the following function that’ll compute the overtime for a given day.
(defun compute-overtime (duration-string)
"Computes overtime duration string for the given time DURATION-STRING."
(let (minutes-in-a-workday
work-minutes
overtime-minutes)
(defconst minutes-in-a-workday 480)
(setq work-minutes (org-duration-string-to-minutes duration-string)
overtime-minutes (- work-minutes minutes-in-a-workday))
(if (< overtime-minutes 0) (setq overtime-minutes 0))
(org-minutes-to-hh:mm-string overtime-minutes)))
Then, use this in a clock table formula in the file whatnot.
#+BEGIN: clocktable :maxlevel 1 :emphasize nil :scope file :formula "$3='(compute-overtime $2)::@2$3=string(\"Overtime\")"
#+END: clocktable
Hit C-c C-c when you are on the clock table to regenerate it.
You can get the total overtime by summing the overtimes using another formula. But, I haven’t worked that out.