If you do not require the totals you could leave the summary with no measure set up, although that seems to defeat much of the point of a summary. Ar the values for A and B already summarised in your example?
You can also set a formula based calculation for any measure value which is set up in the summary - assuming you have an appropriate version of Monarch - which your profile suggests you do have.
However if the values of A and B are 2 values from the same field (an example would be where you are using 'across' values for a field in the summary) there is no way specify a calculation for the line. However in that case if you needed to subtract one value from another I would have expected the original data to identify whether each original value should be considered positive or negative, in which case a SUM would provide the correct answer.
Alternatively export the results and treat that as a new report source with each value in a separate field from which the calculation can be performed.
Guess there is nothing much I can do as values in A and B are from the same column and yes have used the across function... /b[/quote]Well, it sounds like you need to treat them )or versions of them) as separate data if you need the calculation to be something other than a SUM.
So maybe a calculated field in the Table that looks for B values and makes them negative (for example) but leaves the A as positive. The report on that field in across mode across and the SUM should work (assuming they are values. If they are simply record counts then the idea does not apply.)
Alternatively, but along the same line of thinking, convert the A and B values (if they are values not counts from a summary) into two separate fields in the table (Using the IF() function for conditional processing) and then you can do the calculation in the table ...
There may be reasons why neither of these ideas will work for you but I thought it was worth mentioning them.