PVC box fill calc - internal clamps

I can't seem to find a definite answer to this, so here goes.

I'm prepping to get a permit to do some electrical work, which will entail branching from an existing circuit at a ceiling box to add a few outlets. The existing box is too small to add anything new to, so I need to swap out the box. My question comes in regards to the fill calculations.

This box will have 4 14-2 w/ gnd NM cables and nothing else. Based on my calculations, this requires 18 cu in for the wiring.

I know the calc also requires allocation for any internal clamps. I'm looking at using PVC boxes that have the pressure tabs in them, like this http://www.homedepot.com/p/1-Gang-18-cu-in-Round-Old-Work-Ceiling-Box-B618RR/100404072. Are those tabs considered internal clamps that need allowance in the fill calculations?