Need to Validate total of data on one form with static # on another

[TheAxe]

Hi People!

I have a form that lists all Customer Orders by Order:

Date Product Quantity [Button to open shipping form]
Today UselessJunk 50

When you press the button on any line it opens a small form that you can put in the shipping amounts. You can enter as many shipments as you want. This is because we often ship part of an order at a time.

Amount Date
25 somedate
25 Someotherdate

There is a nice running sum in the form's footer.

Problem is with data validation. I am worried someone might keep entering shipments in excess of the ordered amount thus wrecking the data. I need to stop users from entering data when the running sum of the shipping form exceeds the Order amount on the Order form.

Help, how do I do this.
Any advice would be most appreciated!

 

[TheAxe]

No clue eh.

I tried putting the rule on the total but no dice.
Tried putting it on the field itself and having it do a sum and compare but again no dice.

Its really a problem cause I got 2 other similiar situations now.