Find Percent Area of Polygon inside another Polygon

Hello,

I have ArcInfo 10.0.

I am trying to find the area of a multipart polgon inside another polygon. For experimentation purposes I created a single part polgon called BOX. I then place several circles inside that BOX polygon and saved those as a multipart polygon, CIRCLES. Another thread suggested clipping the Circles from the BOX and calling the output BOX_CLIP and then creating a BoxArea Field in Box_CLIP and joining them on that field. When I try this, all of the values in the fields joined to the BOX attribute table are <Null>. What I am ultimately looking for is the percent area of the BOX that the CIRCLES occupy.

Any Suggestions,

Thanks

Howard Williams