Allocating supply costs in one Accounting string to multiple Accounting strings

Summary: Currently we have a supply room that is used for 2 different departments. Right now all expense is going into one departments budget and we have to manually calculate the percentage that belongs to the other and we do this in a manual journal entry.

Content (required): We are looking into Oracle Allocation functionality to see if we can set something up that will do this automatically. at our go-live we were under the understanding that an allocation would take all expense from one accounting string and move to another. However we only want a percentage of the expense to move from one string to another.