You need some slack

Allow some slack, a bit of looseness, in your scheduling.
Highway planners, for example, know that a freeway can handle an optimum number of vehicles. Having fewer cars means that the road is not being used at capacity. But at that optimum point, if just a few more cars are allowed to enter the traffic flow, everything comes to a crunching halt. With the new metering devices that control access during the rush hour, planners can get a fix on the right number.
The same thing can be done for managerial work.

There is an optimum degree of loading, with enough slack built in so that one unanticipated phone call will not ruin your schedule for the rest of the day, the week or more.