Sum up BOM item characteristic values and display on BOM Header item

Hello All,

Here's my scenario and need expert advices  :

I have BOM items with characteristic to capture Sq ft value for each BOM item . 

Now, I would like to sum up SQ ft value of all BOM items and display Total SQ ft at header .

$SUM_PARTS is not supported in AVC.

Here's the error . any alternative way to calculate  sum of all  BOM items width and disapply at higher level model/BOM

EVCH_HL_AVC_SYNTAX180 $SUM_PARTS is not supported in AVC