Not a bad idea on fortificaiton but I think if you use that method that the 25% should divide the damage by 2 and the 50% would divide by 4. That way the more fortification you have the less damage you take.
However, like the discussion in this thread:
http://www.enworld.org/forums/showthread.php?p=1303874#post1303874
I came up with additional methods to allow fortificaiton to still be useful but not overwhelming. My method only allows fortification to stop one effect of a critical, special effect, sneak attack, or feat/class ability.
I use a method that if roll in your threat range on the initial roll and hit them then you are threatening a critical. When you roll to confirm the critical if you hit you get a normal critical, however if you roll in your critical range again, you get another confirm roll to confirm a special critical. Special Critical are taken from a chart similar to one from Dragon Mag a long time ago and updated for 3E. This chart increases the multiplier, applies specific location damage and penalty etc.
This way critical hits involve more than just damage and makes some spells actually be useful, since special critical can remove limbs, eyes, etc.
If you take a critical and you have fortification and it holds, then the critical is neutralized.
If you take a sneak attack and you have fortification and it holds, then the sneak attack is neutralized.
But if you are hit with a critical and a sneak attack and you have fortification and it holds then you choose one to neutralize.
It just get more fun to watch a player agonize over which to ignore with they are hit with a critical attack during a sneak attack that results in a special critical effect, and the rogue have a special effect like crippling strike. Then they have to decide which one to neutralize. They can only choose one: Sneak Attack, Critical Hit, Special Critical Effect, or Special Ability. All the others still apply no matter which one you neutralize.
Of course this makes combat deadly, bloody and does slow it down at times depending on how many criticals are rolled but then again combats take a while to play out anyway when you have characters with average level now of 35, after 3+ years of play,and most combats involving 2 dozen or more total characters (both sides combined).
IMHO,
RD