For planning on film sets I would use the "this is how long it would normally take to do X if you do it at a normal tempo and no unforseen blockers appear".
For these you add the buffer. Sets that I scheduled were most of the time slightly faster than the schedule, until a blocker appeared which ate the buffer, but was usually resolved without eating it away completely. So at the end of a day we were always either on time or a bit ahead.
In film this is the ideal case, if tou depend on the weather or on certain locations being open, missing your slot can mean that you have to try for a whole week to get it again (or have everybody work on something else and rush them over once it works)
For these you add the buffer. Sets that I scheduled were most of the time slightly faster than the schedule, until a blocker appeared which ate the buffer, but was usually resolved without eating it away completely. So at the end of a day we were always either on time or a bit ahead.
In film this is the ideal case, if tou depend on the weather or on certain locations being open, missing your slot can mean that you have to try for a whole week to get it again (or have everybody work on something else and rush them over once it works)