Currently there is no "measurement over sweep" capability (there's
measurement over corners, but that's not what you want).
If you saved the family data from the monte carlo (on the monte carlo
options form), you can bring up the results browser (from the pulldown
menu), and then perform the calculation at the end in the calculator -
but you can't put a spec on it (yet).
Please contact Cadence customer support if this is something you want.
Regards,
Andrew.