Thank you for your reply.

I created the field x_volume_subtotal in Purchase Order Line Model and I tried to apply the following calculation formula to it: x_volume_subtotal = (volume_of_l'article * quantity) / packaging.

for record in self: 

    record['x_volume_subtotal'] = (['product_id'].value.volume *record['product_qty'])/['product_id'].packaging.qty

You can add new fields in purchase order line where you can store the volume per item (if not defined on product). and add new field for purchase order where you can calculate the volume based on your formula.