Non-stock items on a PAR cart not converting to Buy UOM

unknown

Does anyone have non-stock items on a Par cart where you track them by EACH but have to buy them by Box and have it create the PO with the correct Quantity for the buy UOM?

We have several items that have to be setup to track at Each but buy is Box. There are 12 in a box and we need to order another box when it gets below the Par value of 12. If I put the count in as 11 left, I get errors in MSCM that it can't convert to the Buy UOM.
I thought there must be a way to have it round up to a minimum value when it goes to the PO.
To make this more complicated is when we have items that need to keep 3 boxes on the shelf and have the Par be a multiple of the number of eaches in the box... then you have to calculate how many are left and make sure its divisible by the buy UOM to get it order correctly.

I know there has to be a better way this this. I keep thinking this should be an Inventory item but the dept instances it is a non-stock.