Is it returning blank or 0? When you take customer_nbr out of the viz then none of your ELSEIF statements can evaluate to TRUE, so I would have expected it to fall down and out through that final ELSE branch and return zero. But either way that calc won't give you the totals without that field being somewhere in the viz.
I assume you're aggregating this because the [total_tickets_2014] field is an aggregate, right? I suspect there are other ways to work this out. If you post a package workbook with some sample data we'll be able to better help you.
Even with customer_nbr in the viz, it still returns 0 as grand total.
And yes that's correct , the [total_tickets_2014] is an aggregate.
I will upload sample dataset in 30 minutes.