You're correct in your assessment that this isn't possible within a summary. The solution is to export the summary and then use that export as the input for another Monarch process in which you can add the calculated field using the If() function. If necessary you can then build another summary with this "final" data set.
Can it be included in next version of Monarch?
I would suggest that you add the idea to the "Future features for Monarch" thread listed near the top of the Monarch forum thread list.
However, the nature of Summary reporting may mean that the requirement is not well met by such an internal calculation at the summary level. Summary filters, for example, might produce rather variable results?
Is there any way you can perform the calculation for each record in the table and then use the the resulting calculated field value in the summary?
I will put the request on the "Future Featurs" request list.
I had included this calculation in the table, including the "If" statement. This "If"-statement is also needed in the summary because it requires the division of two sum formulas and an if-statement that when the denominator is zero or less the outcome should be zero.