Rating based on ship unit length

 Hello,

I am trying to setup a rate structure that will support the rating of shipments based on a range of lengths for a flatbed equipment.  So if 0 - 12 ft long then use this rate, from 12-24 use this and so on..There is a rate basis for ship unit length which works perfectly but as soon as I incorporate an order that has more than one ship unit it does not work.  I have also tried the length mins and maxes on the rate offerings and then just have a rate offering for each of the three rate records