How does the Projects Allocations functionality works for allocating Shared Services costs?

Summary:

We have Shared Services costs that we need to allocate to several Business Units/Cost Centers each month either based on usage or weighted percentage. This is a manually process and maintained via an Excel spreadsheet. Project module has a Projects Allocations functionality but can someone share how you can set up the monthly allocation distribution rules for based on Actual Usages what changes each month or by fixed percentage against various Projects?

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