Percentage Based Billing for Resources

Hi Gurus,

We are using project management and Project Billing.

We have a requirement to bill the customer on percentage basis through PJT .

For this i have created some Deliverables against task...but i saw in Deliverables "invoice amount " and 'Revenue' amount' are available for entering amounts if i will select 'Billing'.

Selected deliverable type is 'Milestone-Billing'.

But we are maintaining the amounts against Task (Resource:Employee) in Work plan.

How i can handle Percentage for each task for generating the Invoice amount.

Thanks

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