I'm no OEE expert, but I checked with our production manager, who is, and he commented as follows:
We are basing our TART on scheduled work hours.
It depends on how you want to use the data. If you are not machine capacity constrained and want to use it to measure how well you are performing and improving against planned work time the actual "schedule" fits. If you are machine capacity constrained and are trying to justify capital expenditures you need to measure OEE as a function of total time which includes all the time the machine is not manned.
Example below puts it in terms of reliability (scheduled production) vs. equipment utilization (24/7/365)
Percent of (scheduled production - reliability) or (calendar 24/7/365 - equipment utilization), that equipment is available for production.
The new KPI that seems better suited to 24/7/365n is TEEP. See link above.
The pph goal in TEEP is usually defined as the maximum theoretical capacity in parts pph. No set-up, downtime and running at machine rate continually. In TEEP you usually calculate based on the 24/7/365 schedule.