What does "below grade" mean?  I picture a basement.  We don't have basements here, because of caliche. (You can look it up.)  But I have lived in basement apartments, and they cost a lot less to cool than upstairs.  Also, they can cost a lot less to heat if the radiator pipes are exposed and below the ceiling and not well insulated.  So sometimes you pro-rate based on percentage of square footage, but sometimes you pro-rate by guess and by golly.