I suspect from the careful and concise wording of your question that you're aware of this and trying to specifically get white space for the .STM file. The answer to this is that you'll need to take your Exchange store offline during a scheduled maintenance interval, and then run ESEUTIL /MS on the store in question.
Using "F" to represent the value for "Free" space, and "WS" to represent the value for "white space" in the .STM file, utilize the following formula to calculate the size of the white space in kilobytes in the .STM file: WS = F * 4. Obviously, if you want to convert this to megabytes, divide by 1024 and so on.
I think you'll see why most people are happy to use the event logged in the application event log for an idea of overall white space.
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