Dynamics Corporate Performance Management explains Adding a column that divides an amount by the number of reporting periods in Management Reporter.
Adding a column that divides an amount by the number of reporting periods – Dynamics Corporate Performance Management – Site Home – MSDN Blogs
Install, Upgrade & AdminMicrosoft & the GP Ecosystem
Archived post from 2013. Dynamics GP and the links it points to have changed since; check current documentation before relying on version-specific steps.