You are correct. You should subtract start time from end time to be accurate
Format is okay, but doesn't take into account differences greater than 59 minutes. What if there was 1 hour 40 minutes difference? 13:50 end time and 12:10 Start Time would return 40 minutes if you use FORMAT as specified even though 100 minutes has passed. I assume they want the value of 100 for that difference.
Assuming [Start Time] and [End Time] are date/time fields from the form bound table record, I would do this instead:
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dim iTotalMinutes as integer
iTotalMinutes = ([End Time] - [Start Time]) * 24 * 60 'Note that dates are count of days when used as numbers
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