In Shadowlands, Blizzard added diminishing returns to your secondary stats:

From 0 to 30%, there's no penalty.
From 30% to 39%, there's a 10% penalty.
From 39% to 47%, there's a 20% penalty.
From 47% to 54%, there's a 30% penalty.
From 54% to 66%, there's a 40% penalty.
From 66% to 126%, there's a 50% penalty.
You can't get more than 126% from gear rating.

This addon calculates your true secondary stat values, how much was lost to diminishing returns, and how far into each bracket your stats are. Additionally, this addon will try to calculate the true value you will gain from procs and on-use effects that grant secondary stats.

Enjoy!