Summing only certain items in a board

I am building a holiday tracker and could use some help. I have been using for about a week now so am a complete novice.

I have a board where new holiday requests are entered via a form. if the leave is approve they move to the relevant group, within the same board, based on the leave request type (holiday, sick, compassionate, etc). I need to be able to sum the total amount of leave days/hours per requester in order to track remaining leave balance - the list will contain leave for every employee.

I have searched everywhere but cannot find the answer to this.

Any guidance would be greatly appreciated.