how to show a correct ratio for cumulative period

Hi all

I need some help for creating a EPM planning form. We are trying to list a ratio such as "sales amount per sales person" . It obviously works fine for single month though not for cumulative period. Could anybody help us to realize correct calculation? I've been looking at member definition but no luck so far.

Dim Member Definition:

a. Total sales amount (directly stored),

b. Number of Sales person (directly stored),

c. Sales amount per sales person (calculate as a/b)

Result:

Do we need business rule to realize this ER? Any advice would be appreciated.

Regards,

Aran