Volume based pricing on PO

Dear All,

We have item with price breaks,
for e.g.  Item: 1000006308
0 to 10: 25$
10 to 9999: 20$

I am trying to set volume based pricing so the PO should pick the price from price break on total volume.

PO line 1.1 1000006308 qty :5; Price : 25$

      Line 1.2 1000006308 qty : 15; price : 20$

Instead it should pick the price on total volume i.e. total PO qty for the item is 20 so it should pick price 20$ on both the lines.

I am trying to set modifier to achieve this with modifier type discount list and when I select modifier level as “group of lines” in pricing phase there is no pricing phase by name “PO list line adjustment”