Hi Jeff,
You can use the standard macro function MAPE() in a macro to get the MAPE values. You can check 9AEXAMPLES macro book (t.code /SAPAPO/ADVM) for an example. The macro name is MAPE.
MAPE( X1 ; ... ; Xn ; Y1 ; ... ; Yn ) returns the mean absolute percentage error for the values X1, Y1 through Xn, Yn. The arguments can be numbers that you enter in Operator/function dialog boxes, as well as cell(s), row(s), column(s) or area(s) from the planning table or the auxiliary table.
http://help.sap.com/saphelp_SCM700_ehp02/helpdata/en/1d/310ea7d0ee11d4b2e40050da385632/frameset.htm
Thanks,
Rajesh