Think of the calculated measure as a pseudo-cursor. When iterating, it needs to use the member it is currently on (in the iteration) for the comparison to work. Try to introduce the "currentmember" function to achieve this, similar to the following:

[Placement].[Placement Brand].currentmember.MEMBERVALUE = [Metric].[Brand].currentmember.MEMBERVALUE

If you want this to work even if you're not using the dimension on an axis of your query, you'll need to step down to the lowest level by using the descendants() function. Give that a try and post your results if you don't mind.