Here is a great article on unit testing coverage written by my fellow Microsoft MVP Jeremy Clark. Microsoft actually told me that a good coverage goal should be 75%.
https://jeremybytes.blogspot.com/2016/08/code-coverage-should-not-be-goal.html
Strive For Good Unit Testing, Not 100% Coverage
dotNetTips.com 
I don’t recall the exact quote form Martin Fowler, but it was something like: “Imperfect unit tests that are run continuously catch more bugs than perfect unit tests that never get written.” The beauty of unit tests is we can always add more when we find that they missed a case.
Whenever I find a bug, I’ll add a new test to check for it. It’s a great way to improve a test suite.
Here’s a similar quote from Martin Fowler (from Refactoring): “Don’t let the fear that testing can’t catch all bugs stop you from writing the tests that will catch most bugs.” (I use this in my presentation: http://www.jeremybytes.com/Demos.aspx#UTMMF)
That’s probably the quote I’m remembering. It was a lot of years ago. Thanks!