There I was again. A new set of items in need of gems. And like always I had the dilemma of what stats to go for. Not being a theory crafter myself I took a journey to elitistjerks.com to get some hints on where to go. After some reading, containing things I concluded is math, I found a lovely graph that suites my needs perfectly. The dodge/parry relation - what to choose to minimize the diminishing returns. All you do is to check the dodge% in your tooltip and match it with the graph. It is stated that you should check the dodge% from your gear, but I don't know if you need to subtract your naked dodge or not. For the moment I don't and just take the % in the tooltip.
If the graph is 100% accurate is not known, but it is better than anything I can come up with myself.