You'll probably want to use a combination of the SCOPE assignment operator together with the LEAVES function and "THIS" statement instead of using the IF function and testing the coordinate's position.

Calculated members do NOT influence the base value of a parent member, but cell calculations using the SCOPE assignment operator can cascade upwards. 🙂

Also, SCOPE statements are usually better performing than IFs. Hope this helps.

EDIT: You must also check for the empty condition using ISEMPTY() to avoid division by zero. Check out this blog--more specifically the PercentOfParent calculation:

http://sqldusty.wordpress.com/2012/02/28/ten-mdx-calculations-for-your-cube-part-2-2/