I don't like poor logic or faulty math. It's nothing against you personally.
Also in some ways you're also taking the wrong approach. You say one goal of this addon is to help people reforge or gem efficiently? Well have you considered this...
The portion of dodge effected by deminishing returns comes from dodge rating (and agility for druids).
The portion of parry effected by deminishing returns comes from parry rating.
Since the agility portion only applies to druids and druids don't have parry, they have nothing to balance in order to minimize deminishing returns.
Since all other tank classes only gain deminished effect from the rating gain (note that strength is converted into parry rating before this calculation occurs) you only need to balance the ratings between the two in order to minimize the effect of deminishing returns. In other words, when buffed parry rating is equal to dodge rating, deminishing returns is minimized. You don't even have to actually calculate the percentage before deminishing returns. In the case of warriors they will favor parry higher intentionally because of hold the line but if all you're doing is calculating a point to minimize deminishing returns then all you have to do is determine the difference in ratings.
I have considered dropping the Druid part of it for the exact reason you said, but I didnt want them to feel left out. Like you said its basically useless for them except to check their stats, at which point spreadsheets and MrRobot do a better job out of game. Similarly the Diminishing returns percentage is not needed, but I feel like it will help some people realize that they are losing a somewhat significant portion of avoidance to DR.
I could make a 'Lite" version of the addon that only shows the Buffed stats and none of the extra info that most people will likely ignore...
You need to stop worrying about light versions and GUIs and make the numbers work.
Once you have a working version, think about expanding.