Planning for future time off in project schedules

Right now, when planning a project, we have a global admin task for time out of the office that has a daily value - say .5 hours - for example. This helps account for future sick, appointment, vacation time at the beginning of a project before people know what days they might be out of the office.  My question is: Is this the best way to handle future out of office situations to minimize the impact to project schedules, or is there a better way?

Thanks.