A chart published by Brownells in 2001 gives constrictions for each gauge. They vary as smaller bores take less constriction to make choke.

The 20 Gauge is shown as:

Skeet .004
IC .006
Skeet 2 .009
Modified .014
Imp Mod .019
Full .025

Other charts seem to use the twelve gauge constrictions for every gauge, which is misleading.

My experience with several 20 bores supports the Brownell's chart.