PO approval using backup approvers

Hello,

I am new with PO and I have been asked to change the current hierachy, which is very basic, but now some changes want to be made.

First level of approval, Purchasing chief will approve until USD$ 1000.00, but in his absence the accountant chief will approve.

Second level of approval, CFO will apprve until USD$ 2000.00, but in his absence the accountant chief will approve.

Third Level, General Manager will approve from USD$ 2000.00 to N USD$, but in his absence the CFO will approve.

I know I can use the supervisor method or the hierachy method, but I am not sure about the backup approvers. We are doing the PO directly not using requisitions, and R12.1