The only readability measurement tool I know of is the Lix scale. 

The formula is:

The LIX readability formula is as follows:

LIX = A/B + (C x 100)/A, where

A = Number of words

B = Number of periods (defined by period, colon or capital first letter)

C = Number of long words (More than 6 letters)

Here is a Prezi I made about a year ago profiling readability formulas.

https://prezi.com/divwobi7blxf/readability-measures/

here is different methods for RF:

http://www.ideosity.com/ourblog/post/ideosphere-blog/2010/01/14/readability-tests-and-formulas

also Flesch-Kincaid reading ease and grade level.

You can also use Lexile and Coh-Metrix, which give readability information.  I've found those to be pretty helpful.