Acrobat formula (for PDF Expert) that computes time difference - repost

We're creating a time sheet (attached). We need a single formula written so that the circled value equals (((A-B)-.5)-C). In other words: ((StopTime-StartTime)- a half hour) less the sum of column totals. What we've tried works fine if hours are whole, but partial hours is not computing properly. Just for reference in writing the formula, here are the field names we're using: Circled value= text29g A= text 26n B= text26 C=text29