Grand Total to Show as Average instead of Sum

I'm trying to get my Grand Total for Revenue as an example to show as an average instead of a sum in a Mega Table. My Revenue calc is below.

SUM

(CASE

WHEN 

(`Source` = 'Order Items' AND `OrderItems.Type` = 'SpaceOrder' AND `Rank_DateRunOrderItems` = 1) AND
`Reservation.State` != 'Canceled' AND
`Enterprise.Name` IS NOT NULL

THEN

`Amount.NetValue`

ELSE

0

END)