Staff numbers are the wrong basis for the interval
An interval calculated from office headcount will be substantially wrong where the washroom is also serving people who have come into the building for another reason. The additional traffic is invisible in any staff figure and it does not follow office hours, so the facility can take heavy use at times when the office itself is quiet or closed.
Setting the interval requires an estimate of the total load rather than the office component. That does not need to be precise; it needs to be honest about the fact that the office is not the only user. Where the building can indicate visitor numbers or trading patterns, that information is far more useful than the tenancy's own headcount.
