Credit to Audit Services for figuring this out. Typically, the Tableau folks tell you not to bother with adding a column for percent of total to your data, since this can be easily created using Quick Table Calculation. This adds a custom calculation from the source measure you can use like any of the other measures. It also allows Tableau to adjust percentages automatically vs. locking in a certain number of rows with fixed percentage values.
Audit Services wanted to create a bar chart which represented volume count along the measure axis, but add as the Mark Label what that count represented for percent of total. We tried different ways of applying the calculated field for percent of total, but could not get it to work.
We went back to their original source table in Excel and added in fixed percent columns. When connected to Excel, this causes Tableau to recognize the percent column as designated measure instead of a calculated field. That made the difference and we were able to use both in the display. Don’t understand why calculated fields vs. a designated percent column in the data would make a difference, but it this case it does.