Measure your models then buy the trays!
Find out the dimentions of the trays and map the positions of the models on paper remembering to leave space for foam inbetween the models (I think sabol trays come in 1cm pieces).
Example 1:
One tray worth of carnifexes
My tallest carnifex is 3" tall. I'll buy 1x 3" tray.
Example 2:
Two and a half trays worth of gaunts
My gaunts are between 1-1.5" tall. I'll but 3x 1.5" trays.
Consider laying some models on their sides and some standing up.
With come careful planning you can really cram them in there!
Simple really.
Or try this Army Transport Planner!!!
http://keallen.dyndns.org/warmachine/TransportPlanner.html