The wise scheme, as has been pointed out, is to adjust predictions during a project. If the task that was initially planned to take a day is routinely taking 2 days (or 2 hours), adjust future plans accordingly.
Higher-level managers are sometimes unhappy with this scheme, while middle-level ones value the accuracy of such predictor-corrector schemes.
This gets us into a related topic of the depth of management structures, and I think the military scheme (of each person directing a roughly fixed number of persons, so the number of levels is proportional to the log of workforce size) might be worth considering.