How can we take out public holiday hours from calculation of total hours per day?

Hi,

We have a custom fast formula, where we would like to exclude, in the calculation of total number of hours per day, the hours booked on a public holiday.

ex: if a person books 4 hours + other 4 hours as public holiday (in the same day) => we want to get a total of 4 hours (only the ones he worked that day).

Found this function to get total number of hours:

wk_hrs_total_day  = get_wrk_num (ffs_id , 'wk_hrs_total_day' , 0, 0 ) 

But how can we exclude the hours booked as public holiday? As the above is getting a total of everything .