Tables :: Auto-Calculate Field From Other Table Values

How to set up my tables as I'm just starting off with setting my database up.

I'm doing a simple database to track the purchase orders (PO) I am managing. Each PO has a PO Number and an Original Value. POs may have multiple amendments which would change the PO value. I would however like to keep the history of the PO original value and all different amendments.

So I created two tables:

tblPO:
ID
PO Number (Number)
PO Original Value (Currency)
PO Sum of Amendments (???????????)
PO Current Value (Calculated = PO Original - PO Sum of Amendments)

tblPOAmendments:
ID
PO (Lookup from tblPO)
PO Amended Value (Currency)
Amendment Date (Date/Time)
Amendment Desc (Text)

Now the two, million dollar questions are:
1.) Is this the right table structure to use.
2.) How do I go about calculating the Sum of Amendments field?