With a background in traditional project management, we use the same principles in film production as in other types of projects.
We start on a high level with five sub-processes that covers everything from story development to selling of rights and red carpet.
For each process we do a work breakdown structure that describes all activities and tasks needed to finalise each sub-process. To reduce to big uncertainty in the estimation, the rule of thumb is to have activities that are of the size of 50-100 working hours.
Example: We have a team of 20 persons on set, both cast and crew. Therefore, we need to plan their time in chunks of 50-100 hours for them as a team, i.e. 2.5 to 5 hours per working day. Double number of cast and crew, then you get half time for each time slot, e.g. 1 - 2 hours per working day.
Next question is then how long time does a scene take? It depends is the normal answer, but this not good enough if you want to finalize the production within in time and budget. You need to be able to tell how long each scene takes before starting shooting, and also explain how you got this figure.
The bigger uncertainty, the bigger risk with that scene. If you have a huge uncertainty for all scenes, then the risk with the whole production is huge.
Unless financiers put in more money and/or all crew & cast work extra for free, the result of lack of planing is missed shots and/or less quality.