A nice article by Jessica S. Ancker notes why you should not rely on the Flesch-Kincaid scale (or any other scale) for determining the readability of text, on-screen or otherwise.
To get a quick idea of why, check out the following passages and guess which one was determined to be easier to read by Flesch-Kincaid:
DISCLAIMER: WE ARE INVITING YOU TO BE IN A RESEARCH STUDY BECAUSE YOU HAVE A TRAUMATIC BRAIN INJURY. A TRAUMATIC BRAIN INJURY IS AN INJURY TO THE BRAIN CAUSED BY SOMETHING THAT HIT OR SHOOK THE HEAD.THIS CONSENT FORM EXPLAINS THE PURPOSE, RISKS, AND BENEFITS OF THE STUDY. THIS INFORMATION MIGHT HELP YOU DECIDE WHETHER TO BE IN THE STUDY. PLEASE READ THIS FORM CAREFULLY. IF YOU HAVE ANY QUESTIONS, ASK YOUR DOCTOR BEFORE YOU MAKE A DECISION.
We are inviting you to be in a research study because you have a traumatic brain injury. A traumatic brain injury is an injury to the brain caused by something that hit or shook the head.
This consent form explains the purpose, risks, and benefits of the study. This information might help you decide whether to be in the study.
Please read this form carefully. If you have any questions, ask your doctor before you make a decision.
Answer: They were given the same readability grade.