When writing a code you should do it so that the time required for your understanding is minimal. This is the most important metric for having readable code and is the foundation of the fundamental readability theorem.
The longer you spend, the lower the readability of your code, and that's a sign that it needs readability improvements. An unreadable code today may be your loss of productivity in the future. What you do today, with complete understanding, in about 2 years when you need to do maintenance, may be unreadable even to yourself, and this will cause you to spend precious time trying to understand what you have written yourself.
So what are the interesting ways to apply this theorem and measure the readability level of the code?
It is not desirable for answers to this question to be based on opinions, but rather on facts, references or specific experience.