If you are going to take some days for personal travells before or after your Mission and want to deduct those days from the "Per diem allowance", you can follow the instructions below.
1) Go to Orders > Missions.
2) Click on the actions menu of the Mission header, followed by Add line. Alternatively you can Edit... an existing one.
3) Create or edit the respective Per diem allowance line with all the details .
4) Near the bottom of the facebox, there will be an option to Exclude days on personal leave. You can click there and fill the number of days you wish to exclude.
5) The summary table will update the values automatically with the new expected value of the Per diem allowance.
6) Fill the facebox with the remaining of the information and click Add line or Save.