Set up a Weapon Area Goal. In the Debrief, give the percentage or ordnance falling in the designated area as the success condition. 100% means all the ordnance dropped landed in the area (even if this is only 1/1!)
To be save, you might use >=50 and/or make the box pretty large with the ATTACK waypoint right in the middle of the box to allow for drops right at the WP.
Recluse
P.S. The Weapon Area Goal is a special case of the AREA GOAL which lets you specify the weapon that must be dropped (e.g. Captor or Quickstrike mines)