The timing of some future expenses can be predicted with considerable accuracy while others are uncertain. Something that is likely to need maintenance about every 20 years might be scheduled for 7 years hence because it was last maintained 13 years ago. However, it might turn out to be opportune to get it done with other work at 5 years or it might be still in sufficiently good condition when you get to 7 years that you revise and update the plan to push that item out another 5 years.
It is more important to have an adequately funded plan that covers everything than to be overly pedantic about the detail. I would also recommend having an explicit ‘contingency’ component so that if all goes to plan, there will be some extra in the bank to help you cope if something unexpected happens. That could be an expense that was completely off anyone’s radar or it could be a known expense that suddenly needs to be brought forward.