If you are wanting to add a new rule to your report through the Conditional formatting function where you are wanting to compare 2 columns, and the drop-down box below doesn't fit your criteria. Then add a "Helper Formula column" would be your best bet.
1. For example, If "Quantity on hand" less than "Min Stock Level" (separate column), fill in red.
2. Add a formal column in the report, which I have named Rule. (
)The below IF formula can be used for any sorts of comparisons -( the below say If "Quantity on hand" is smaller than "Min Stock Level" then insert Y otherwise make it N)
3. Then you can add a rule, show below.