There is no such value that is calculated internally, which could be scheduled.
However, I figured out a workaround that you can use if you do not otherwise use the Low Ceiling Area Reduction fields in the Options > Project preference > Zones Dialog.
Let us say your Zone Height is 2700 mm. Set the Low Ceiling Reduction values so that 100% below 2700 is reduced from the Zone Area.
Now, if you Update your Zones, then go to the Zone Settings Dialog, you will find that the Reduced Area field now returns that top surface of the Zone after the Roof cut.
In the following screenshot, you can see the result. I have drawn a Fill on the Floor Plan and displayed its area. As you can see it returns the same value as the Reduced Area and Calculated Area fields.
So, with the trick, you can schedule the Zone Area with the Measured Net Area field, and the Zone Top Area with the Reduced Area of Calculated Area fields.
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