Personally I found that my dodge/evasion rate increased as my defense increased

Ya, thats how I figured defense played a role. GNW Neji was the same way. When I swapped my mood from +hp to +def I dodged alot more often the closer my defense was relative to my opponents attack.

Theres probably a few factors that are involved in the formula. But I know for a fact increasing def mood 45% on my neji dramatically increased his chance to dodge when I was fighting in ninja exam. (Think it was the 105 struggle where I needed to dodge almost every attack in 1st round and 2nd round to win)