Rate based on Refnum Value

[nate]

We are trying to base a rate on the value of a refnum: the number of shipments on a BL (or MBOL_COUNT in our config). I see the multiplier Shipment Refnum Value as Number, which seems to be what we want - however, I'm not able to define the refnum qualifier on which it's based (MBOL_COUNT). Is there a better way to accomplish this rate structure?
Thanks!
Nate