So, if I understood correctly, you want to convert 1/2 of all types of AC bonus to DR, right? By raw it's 1/2 of armor, 1/5 of natural armor and the others are left alone; also enhancement bonuses explicitly don't convert to DR (thus a +2 chain shirt would be +4 AC and +2 DR, not +3/+3). I'd rather we go by RAW here, as I can't really see a realistic reason you should translate bonuses like deflection, dodge or even worse dex into DR.