The Account Allocation form can be opened from Accounting menu > Account Allocation. When the Account Allocation selection form opens, select the Main Account and it will bring you to the Account Allocation form where you can set the Component Accounts with its corresponding percentage rate. You can break down the Main Account to as many account allocations or component accounts as needed as long as the total would equal to 100%.
For sample illustration on how this form works, see Accounts Allocation.