Sorry I couldN't be of more help.
By the way, I looked at it a bit closer and it looks like that it decides what the Floor will be based on which Story more of its vertical height falls to.
So if you have a 1000 mm high Window and it vertical height is 500 mm or more above Story 2, the Floor field will display that instead of Story 1.
So it is not in which Story's vertical range the Sill falls to, but where more of the vertical height falls to.
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