OK, instead of doing this as a calculation, we need to use 2 shipping rules and one charge rule.
1) On the shipping tab create the base rule:
trigger: Total Number of items > 0
calculation: Flat Rate:
$_SESSION['ShipZoneBase']
2) On the shipping tab create the incremental rule:
Trigger: Subtotal Of Column Total Weight > 10
Calculation: Flat Rate:
floor(($this->TotalColumn("TotalWeight") - 10) / 0.5) * $_SESSION['ShipZoneInc']
3) On the charges tab, create the Fuel Surcharge.
triggers: Total Number of items > 0
Calculation: Flat Rate:
$_SESSION['ShipZoneFuel'] * $this->GetShipping()
