Deviation factor on master item

Hello

First i create an Item Pencil(Primary uom=Piece and secondary uom=box)

conversion is 1box=10Piece

Now i enter PO for purchasing ITEM piece in primary UOM (10 Pieces x2=20) but secondary UOM box is also mandatory field in PO......while i m not purchasing pencil in Secondary box,so what the data i have to put in secondary UOM?it should be 1 box when i take 10pieces in primary UOM?but conversion is not working bt the above conversion is correct