My calendar has a lot of red-entries ( meaning they are past due ). I have been under-estimating the efforts ( implying time) needed and set hard to achieve deadlines. I know estimating time required for tasks is a difficult skill and it takes experience. But it is an essential skill in life to improve your productivity. 100% is impossible because it would require considering so many factors and require so much time to estimate. A skilled manager with several years experience would get 80%-90% ( a rough guess ). If I can get my estimates right 51% (simple majority) of the time (i.e, get the tasks finished within time), I would consider this goal done.