Decided to pop over into this thread - I'm the one who was doing all the math on Tankspot.com. From what I discovered using brute-force calculation of the best possible avoidance given an arbitrary number of points to spread between dodge/defense/parry rating (with 304 bonus agility as a paladin), the following rules should govern gearing for best avoidance in 3.2:
1) Stack defense rating until you reach a number that has a minimum of wasted in the defense rating -> defense conversion. (728,787,846,910 and 969 are the best possible ones to hit).
2) Stack dodge/parry rating from there so that the post-DR portions of your Dodge% and Parry% are in a 1.875:1 ratio
3) If your total dodge+parry rating is getting anywhere near your defense rating, try to hit the next good number of defense rating (while adjusting your dodge/parry rating to maintain #2) as the diminishing returns on miss will be such that you will notice a slight difference.
4) If you can't hit the next good number of defense rating, don't worry - keep stacking dodge/parry in the aforementioned ratio and you'll be fine.
You can look at the tankspot.com post which Jere posted above to get to the plots, but if you want to look at the raw data,
Here is a google docs spreadsheet of the maximum possible avoidance at each total of rating points (calculated by checking the total avoidance of every combination possible). Aside from listing just the ratings and total avoidance, I also made a column with the post-DR percentages of dodge/parry/miss, and the ratio of post-DR dodge% to post-DR parry%. You'll notice that every single ratio hovers around 1.875:1.
Also, in regards to 1), I am not advocating going out of your way to hit those five exact numbers. Any defense rating amount with <10% wasted defense skill should be indistinguishable from those to two decimal places. (see the tankspot.com post for a plot and further explanation.) Also, once epic gems come out and we're gemming in 10-defense rating increments, you won't be able to use gems to shift toward a better defense number easily by gemming since the steps up a defense rating line are almost all in 5-point increments.
I believe if you run the numbers on your own simulation, though, you'll see that the closer your dodge-parry ratio is to 1.875:1, the greater your avoidance will be for any given point spread.