I've done this before using LODs I believe. Im not sure if its the best way to go about it, but

'SUM({FIXED [filters/fields applied]: SUM(IF [filter_field] = X then [Y] END)})/SUM({FIXED [filters/fields applied]: SUM(IF [filter_field] = Z then [Y] END)})'

should result in what you're looking for.

You can embed IF logic inside your SUM() function.

SUM( if filter_field = X then [Y] END )  / SUM( if filter_field = Z then [Y] END)