Am sorry but in my knowledge of schedules, what you're seeking is not possible yet.
Parameters are defined for project elements only and you're trying to use a "result" of a schedule in a formula of parameters. This just can't work
I hope someday AC Schedules will include some basic spreadsheet function such as the one you look for.
But what you can do is create a new parameter, let's call it "Unity", and define as "Units" for your windows, then make it appears right next to the Quantity field.