I'm not sure I understand what you mean, "with the math"... But here is how it works. flanking adds a withdrawl chance %. Withdrawl means that if a unit rolls while attacking durring combat that it is killed (looses), it will then throw a withdrawl roll. So lets say you have cavalry with a base 30% chance withdrawl, and flanking 1, giving it a 40% withdrawl chance. And let's say this cavalry attacks a Machine gun, and like as expected, he does some damage to the MG but the rolls have him loose the combat, he'll then throw a "saving" roll with 40% odds to withdraw from combat, if the roll succedes, you get a still living, but very injured cavalry, if it fails (60% chance in this example) the cavalry is killed.