UPH should be Total Units / Total Hours of work
Not sure but is that just the Operation Sequence being used in your formula?! If so, you may correct it similar to below:
[TotalUnits] / ([EndTime] - [StartTime])
Also, if the EndTime and StartTime columns are in DateTime type, then the value you get for (EndTime - StartTime) will not be Hours. It might be the least component (probably seconds). Please check.