First thing to do is to calibrate your pedal.

http://line6.com/support/docs/DOC-2223

Then, set the min/max levels on the pedal to a more narrow range. This will take some experimentation, but the idea is that the more narrow the range, the less difference occurs with minor modifications in depressing  pedal.

For instance, with a min level of 50% and a max level of 75%, the entire range of the pedal will only move you through 25% of the potential range, equivalent to the 1/2 to 3/4 range when the min is 0% and max is 100%.

This a preset-based setting, not global.

I too use Min and Max on the expression pedal to limit the range ...  but with gain rather than volume.  Just be aware that iby fixing the max and min at anything other than 100% and 0% respectively will mean that you either cannot hit max possible volume or total silence respectively.  Still, this is probably the only approach that will help you here.

Personally, I don't really use the volume pedal and prefer my guitar volume for swells.  However, I understand that the Pod expression pedal always generates a linear change in the parameter that it is controlling.  In some cases this is fine but this is not good for a volume pedal, where a logarithmic change is required .

More discussion about this in the thread here --> http://line6.com/support/message/359010#359010