How have you structured your data?
You have an invoice table and an event table, but how are they related?
How do you record the total tickets sold on a given invoice? Is there a line items table?
Does Sum( Event::TotalRegistrations ) compute the total of all registrations or just one invoice?
In response to your Private message providing more detail:
Define a field of type calculation in Event Line Items as If ( Event Invoice::date Paid ; Qty )
Then take the sum of this calculation field to get the total of all ticket quantities for only paid invoices.
Thanks SO much - I did what you suggested and it works pefectly!
Now to take my first crack on some Conditional Formatting to 'gray out' a few fields whenever a certain different field shows a certain value. I expect trouble so get ready. :)