org-table - managing my badge times

Question

I have the following org-table

datum	werk start	pauze start	pauze stop	werk stop
12/02/2024	09:48	12:45	13:14	17:22
13/02/2024	08:01	12:06	12:31	16:38
14/02/2024	07:38	12:12	12:38	16:48
15/02/2024	08:11	13:11	13:28	17:08

Now I want to calculate how many hours I did per day and see if I did too much or too little in a month.

For this I wrote some helper functions.

   (defun hm-to-minutes (str)
     (let* ((lst (split-string str ":"))
            (hour (car lst))
            (minute (cadr lst)))
       (+ (* (string-to-number hour) 60)
          (string-to-number minute))))

   (defun hm-get-minutes (str)
     (string-to-number (cadr (split-string str ":"))))

   (defun minutes-to-hm (minutes)
     (concat (format "%02d"  (/ minutes 60))
             ":"
             (format "%02d" (% minutes 60))))

  (defun hm-minutes-floor-quarter (str)
    (let* ((original (hm-to-minutes str))
          (minutes (hm-get-minutes str))
          (remainder (% minutes 15)))
      (- original remainder)))

   (defun hm-floor-quarter (str)
     (minutes-to-hm (hm-minutes-floor-quarter str)))

   (defun hm-ceil-quarter (str)
     (minutes-to-hm (+ (hm-minutes-floor-quarter str) 15)))

I also wrote some table functions, however I'm not happy with all the intermediate columns and the result on the last row in the columns I don't need.

datum	werk start	pauze start	pauze stop	werk stop	werkelijk start	werkelijk stop	pauze	te doen	gepresteerd	delta
12/02/2024	09:48	12:45	13:14	17:22	10:00	17:15	00:30	08:00	06:45	-01:15
13/02/2024	08:01	12:06	12:31	16:38	08:15	16:30	00:30	08:00	07:45	-00:15
14/02/2024	07:38	12:12	12:38	16:48	07:45	16:45	00:30	08:00	08:30	00:30
15/02/2024	08:11	13:11	13:28	17:08	08:15	17:00	00:30	08:00	08:15	00:15
					#ERROR	#ERROR	00:15	08:00	-00:15	-00:45

#+TBLFM: $6='(hm-ceil-quarter $2)
#+TBLFM: $7='(hm-floor-quarter $5)
#+TBLFM: $8=$4 - $3;U
#+TBLFM: $8='(hm-ceil-quarter $8)
#+TBLFM: $9=08:00;U
#+TBLFM: $10=($7 - $6 - $8);U
#+TBLFM: $11=($10 - $9);U::@>$11=vsum(@2..@-1);U

I'm most interested in columns gepresteerd and delta.