PO Approval by Item Category

Dear,

I have to setup the purcahse order approval amount limit and amount limit is based by the item category amount Like : Item Catgory from AF:00 to AF:10 = 1000 $

If the PO amount=999 $ and the item which is select in the PO is belongs to the Item Catgory AF:00 to AF:10 . So the PO will be finally approve by the first level approval , if the PO is more then 1000 $ then it should be approve by 2 level approval person.

Due to the above scenarion I setup the item category setup option in the Approval group but system cannot consider the amount which I define with item category and it's tranfer to the second approval in any amount.