To allocate the rental cost to the projects and tasks based on the raw costs of the tasks from the previous month, you need to do the following:

Allocate rental costs once each accounting period by using the “full” allocation method. This method allocates the entire source amount to the target tasks in one run.

Allocate to all eligible tasks and prorate the allocation by the total actual raw cost accrued for each task during the previous accounting period. This ensures that the source amount is proportionally allocated to the tasks based on their relative raw costs.

Define allocation basis by using Actual Amounts with a period-to-date amount class. This allows you to use the actual raw costs of the tasks as the basis for allocation and to use the amounts from the previous accounting period. Reference: https://docs.oracle.com/en/cloud/saas/project-portfolio-management/20c/oapfm/manage-project-costs.html#OAPFM2355951