Measure Behaviour Dependant on Column

I need to create a measure that has different behaviours depending on the column being referenced.

For example for Table 1 Column A compare count as a percentage to Table 2 Column A.

Alternatively for Table 1 Column B compare to Table 2 Column B in the same way but all percentage differences should be inverted (I.e. if the count is higher, and so a positive % difference, inverse that to a negative)

I am struggling with column references, and how to formulate the logic.  Any ideas?