Christina since this will be a couple of table calcs, working without a workbook will be difficult. I suggest you recreate your situation using the Superstore dataset that came with Tableau.
We need two running sums and we need a subtraction. Luckily we can do this with one calculated field within tableau.
RUNNING_SUM(SUM([Sales])) - RUNNING_SUM(SUM([Profit]))
This is using the standard superstore data set but you should be able to adapt it to your own purposes as well.
Thank you Carl. I was not using sum as well as running sum so it wasn't working since it was not aggregated!! Doh!
Sometimes you just need to reach out eh