Ratio to "Parent on Rows" in MDX

Every now and then the issue of computing a generic “ratio to parent” calculation comes up. There is a good overview of the problem in Darren Gosbell’s blog “MDX ratio of current parent issue”. I want to pick up the conversation where Darren left it:

It is technically possible to get "kind of" close doing something like the following using the Axis() function: <MDX fragment skipped> which sort of gives us a  "Percent of Row parent" calculation and this is probably the best you can do, but if you crossjoin multiple hierarchies on the row axis we are in trouble again.

I am not big fan of calculation which depends on the content of axis, mostly because when result of the same expression at the same coordinates differ from query to query (and this happens when expression uses Axis function), the whole caching is destroyed, and as a result performance suffers. However, if there is a real business requirement to build calculated member like that – it is certainly possible, and in most generic way, which will work no matter how many hierarchies are in the row axis. What’s more interesting, it is even possible to do it in pure MDX, and this MSDN forum thread provides different approaches. However, none of these approaches is practical one. The practical solution is to use the following stored procedure: