Since all polymers and additives are very different, it is hard to give an exact estimate of just how little you can put into the EX2 for minimum output. For minumum output, you just need to be sure that the barrel is full for long enough.
You can use the following calculations to figure out the minimum amount for individual polymers:
Hopper Volume: 26 cubic inches
Screw Void: 1.75 cubic inches