Ah, I get how you're rolling up category spending from that line-time table, so having a one-to-one transaction-to-designation record is expedient. Am unsure how I'd otherwise structure that, but a thought is that you could set the spending categorie...

Try this:"Week of " & DATETIME_FORMAT(DATEADD(Date,1-DATETIME_FORMAT(Date,'E'),'day'),'ll')

RE: "I know I'll need AT LEAST one item for every transaction."    ...why not just structure your main table with both a line item amount and split line records, then configure a formula field to use the former when there are no splits or the sum of ...