MDX Calculated query

I am trying to create a TFS report which requires MDX. I am making a report that requires a calculated member. I am not 100% sure how this is done. The member i am trying to do is a difference calculation.

For example:

Table
Sept 1 Sept 2 Sept 3
Actual 0 32 58
Remaining 163 140 132
Difference 0 9 50



The calculation for the difference is as follows:
Actual Effort for that day - (Work remaining previous day - Work remaining that day)
32 - (163 - 140) = 9

Can someone help please?