I just checked and the formula for the bonus is: 3.75% + 3.75% * rank. At rank 99 that would make the bonus 375%.
It is easier to think of the ancient legendaries as counting twice.
The Damage Reduction is 2% per legendary (ancients 4%) and the they sum; to a max of 52% (when all ancient).
You people make my head hurt. I feel dumber having read most of your posts.
It’s exactly the same bonus as the LoN set (as stated above) with the added benefit of providing half bonuses for simply legendaries where LoN does nothing. Yes, you have to rank it up to 99 to match the LoN exactly, but most people can solo 70 on the first day each season so ranking the LoD gem is not only possible but this gem opens up LoN builds earlier than the rings. You no longer are REQUIRED to get perfect gear to play an LoN set.
My only nit is that the bonus should have maxed out at rank 100. I would have given a base 25% + 3.5% * rank. It maxes out the same and makes the gem more useful at the lower levels.
It is also interesting to note that the bonus crosses 1000% after rank 130 (all ancient). There is room to balance LoN builds by upping the max rank for this gem. That is a very good thing.