Just a simple math.round upon damage that would otherwise break legs. This is compared to the value within LegBreakCfg.c and when below that value legs will break as normal, if above they won't.
Percentage change of leg breaking can be adjusted in LegBreakCfg.c
under the value: float chanceOfLegBreak.
Hell knows if this is really the correct way to do it, but fact is it does the job.
Use, Abuse. Do what you like with it, if it interests you.