With the many thoughts going through my head regarding the Imperial Fists, I am at two minds whether they require a special rule. If there was one, it would have to be quite placid as I feel the list structure itself should be the signature of the force. That was until I woke this morning with a special rule

From the stories and tidbits I have read, it is evident that the IF are able to make excellent use of cover for defensive-siege purposes. Without having silly mathematical additions or verbose rules, this could be represented in a more elegant format.
The idea below I think works so I would be interested in your thoughts (rewording may be necessary):
Fortified Defence
When, with at least 50% of the formation defending from cover at the start of an assault, Imperial Fists count a 'drawn' combat result as a win. The opponent is broken with no extra casualties and follows the rules for [insert reference here]. In effect, there is no 2nd round of combat with Imperial Fists who are defending from cover - there is always a winner and a loser.
This rule would also open up the possibility (supplement wise) for Iron Warriors to have a
Siege Specialists rule which would be that they always win drawn combats when assaulting cover - hence the two would cancel each other out. The only time a drawn combat in cover would continue with Imperial Fists is if the Iron Warriors were doing the attacking.
Thoughts?