Feedback Time Clause
One thing that can sabotage even the most comfortable time buffer on a project is a client who takes forever to give feedback on drafts/demos. Unfortunately, this is not uncommon as directors/producers usually have a lot on their plate during post production phase and simply might not have enough time to listen to your cues in a reasonable time frame. If you have the feeling this might be the case with a project, make sure to mention that you need a quick feedback when starting the project, but this of course doesn’t guarantee to actually get the feedback in time.
Another approach would be to request a clause in the contract to receive feedback on any cues for instance no later than 3 days after their delivery. But as I mentioned in another tip a while ago, starting to micro manage everything in the contract might leave a bit the impression to not trust each other on basic working relationship principles, so I would only request such clauses if the client really seems problematic in this regard or has a history of delaying feedback extensively.