Hello Brent Johnson,
Is this article - Resolving "Cannot mix aggregate and non-aggregate arguments" Calculation Error | Tableau Software getting at the issue encountered? You may be able to work around the issue utilizing LOD Calculations because they allow you to compare different measures with different levels of aggregation. here is a good starting point - Top 15 LOD Expressions | Tableau Software
These calculations do not work in every circumstance however they are very useful tools for many situations related to the need for comparisons of metrics at different levels of aggregation.