adding data in fields

I work in the mechanical field currently designing hvac systems. I am trying to use fields to link the amount of cfm put into a room this is fine when there is only one supply into the room, but when there is multiple it creates a problem is there a way to select multiple objects and have it add them together to create the field value

EDIT: I tried using the formula and linking my cfms to a table then using formula to get the sum but it doesnt seem to link adding fields