We should take guidance from the BCEA.
When calculating leave pay, for example, the average earned over the previous 17 weeks. This can give you an hourly/weekly/ monthly rate as averaged.
It removes the decision of regular/intermittent and creates a mathematical average that is directly in proportion.
If person in last week worked 10 hrs of overtime, they can't expect, the weekly wage to be calculated as 55 hrs, for example. Instead 10hrs divided into 17 weeks, equals .7 hrs added to weekly wage. (The commissioner may try escape this particular example as irregular)
Further, overtime that arose from a sudden event, e.g. a strike or catastrophe, could easily be classified as irregular/intermittent. Intermittent would essentially be a bonus upon which the employee has not become dependant.