For Quarterly Reports, it may shorten the process enormously to use the ACD system to produce Totals only (calls, say, or agent hours) and then divide by the number of working days in the quarter, which we compute separately from the monthly acd-reports (all the days with non-zero number of calls offered).
Gate Traffic Report seems able to total the N of Calls offered over a three month period. It produced 4785 offered calls for 81 calls / day. (ACD.xlsm tells me 84. Hmmm.)
ACD Group Report in the agents catalog of reports seems able to total "Login Time" over the three month period. For Q2 FY10 it found 2140:43 hours. Divide by the 59 working days and you get 36.28 (while my the ACD.xlsm sheet found 37; hmmm.).