Purchasing Workflow Customization

But my requirement is such that current flow needs to modified as below:

a)Currently the Routing is as below in steps: 1)BUYER Raises PO and select  "Submit for Approval" and "Forward" from the APPROVE TAB. 2) It Goes to  PURCHASING MANAGER 3) PURCHASING MANAGER Approves and it routes to DIRECTOR OF PURCHASES. 4) DIRECTOR OF PURCHASES Approved it and it then routes to CHIEF ADMINSTRATIVE OFFICER. 5) CHIEF ADMINSTRATIVE OFFICER Approves it and  notification is sent to BUYER.

b) Expecting Routing is Required as below:

1 & 2  mentioned above are fine.

But After  PURCHASING MANAGER approves the request then the PO need to be routed based on the PO Amount.