SSAS - Help Needed

Need help with SSAS I have a cube with a custom calculated measure defined using MDX. Consider the following formula: C = F(X,Y) where X and Y are two measures, C is the custom calculated measure and F is the MDX function. Now for the totals I want? ? ? C total = Average(F(X,Y))? ? That is I want the average of the custom measure. I do NOT want F(X total, Y total). I need instruction of how to define this custom rollup in my SSAS Cube.