Break on Running Total

I am trying to create Packing List(s) based on an individual sales order.

Each Packing List page must be based upon the number of square feet.

I have set up a detail line with
Pieces, Width, Length and a running total of square feet (Pieces*width*length)

When the Running Total Square Feet > 400 then I need to go to a new page and reset the Running Total Square Feet and continue to print the remaining detail, until it again reaches 400 square feet.

I'm close, but I am having trouble. Can anyone help?

Note: Its MAS 90, if that makes a difference.What did you try and what problem are you facing?