What level is your fighter? Does he have great fortitude or an item that improves Fortitude? Is he human?
Cause +4 would turn that 50% into 70% which is really about the upper bounds of where your missed chance should be... and an item and great fortitude would net you another +4 which would bring that to 90% missed. Human 95%. Etc.
True, but as in your last post house rules may need to be balanced with other houserules. If you allow for a net bonus to NADs, it may be prudent to remove NAD boosting feats.
Just to back up some of my words I decided to run a quick test. I pulled up one of my characters from the char builder, an 11th level dragonborn paladin. He's a strength/charisma build, with no special feats or items to boost fort, but otherwise has a big strength and solid defenses. His Fort Defense is 25.
I ran an average of Fort Attacks for 11th level monsters and the attack was +14. That's exactly 50% attack rate on my guys Fort Defense, so the 50% hit rate on high NADs seem right on the money, at least at 11th.
Getting back to the discussion, I'm personally fine with raising weaker NADs as well, just stressing the point that I currently don't think higher NADs are so high they can't benefit for some of these houserules as well.
Also, I'm not a fan of more stat raises to account for defenses. My problem with that is your taking a hammer to the problem instead of a scalpel. Stats affect a lot more than defenses...skills, ability rolls, feats prereqs, etc.
If the problem people have is with defenses, then focus on defenses. Straight up bonuses to those defenses directly addresses that problem, without affecting the rest of the system at all. Its a more elegant way of addressing the issue imo.