Procs tend to go off 5% of the time is the number I hear. It makes some sense, as 5% is the base rate for block, dodge, and parry. If 5% is right, then divide the proc damage by 20 and treat that as the average damage for the proc and add it to the dps. Then compare with the addition for the damage enchant dps boost. If you PvP, remember that the ability to get a high damage burst can be better than a slight increase in average damage- it scares people and makes them heal earlier.