Where Intuition Fails
If one accepts the logic of the foregoing, one would naturally think that a designer would be safe to place black type on a background as dark as 40% or 50%. The reality is that such type is tiresome, even oppressive, to read.
Why is that?
Because white reflects all available light and black absorbs it. Therefore, white is a stimulus to our visual system, and black is the absence of a stimulus. So when we read black type on a 50% background we are seeing a non-stimulus against a half-stimulus.
On the other hand, when we read white type on the same background, we are seeing a strong stimulus against a half stimulus. That’s why it’s inherently easier and why reversed type, even at the minimal cutoff, seems to have better contrast. Even so, the 35% minimum contrast is a useful rule of thumb.
In practical application, the cutoff for comfortable surprinting versus reversing isn’t at the 50% mark, but at about 35%. Does that strike you as an odd number?
It might, until you remember the Golden Section, the Fibonacci sequence, and its modern counterpart, the Rule of Thirds.
The ancient Greeks discovered a particular proportion that kept cropping up in many different places in nature. They called this the Golden Mean or the Golden Section. They considered it a divine ratio and sought to use it in some of their buildings and sculpture.
If you begin with a rectangle built with Golden Section proportions, and subtract a square from it, a smaller Golden Section rectangle is left over. Continue doing this and you will get the diminishing series of squares shown at top. If you swing an arc in each square connecting opposite corners, you will create a Golden Section spiral. 
A very similar spiral is found in nature with surprising regularity.
This same Golden Section proportion is also found repeatedly in the human body. Golden Section ratios are shown here superimposed over daVinci’s drawing of ideal human proportion.
Leonardo Fibonacci, a pre-Renaissance mathematician, discovered a sequence of numbers where each adjacent pair of numbers gave the Golden Section proportion, more or less.
In modern times, photographers and other students of visual arts are often instructed in the “Law of Thirds,” which is that compositions will seem most dynamically balanced if major elements are placed within the picture plane at a location of 1/3 or 2/3.
And so it is with the question of when to surprint reverse. Surprint dark type over backgrounds up to about 35%. Reverse light type over backgrounds from 35% to 100%. In practical terms, there are twice as many colors over which a reversal will work compared to those that will work for surprinting. That’s just a fact of visual reality and essential for good contrast.
Adapted from Logo Design Theory: How Branding Design Really Works