Many people who use readability formulas avoid using the Spache Revised Readability Formula (1974) because, they say, it takes too much time. Others claim that the math is too complicated for them.
Smith, L. L. (1980). Rapid Computation of the Spache Revised Readability Formula. Reading Horizons, 20 (3). Retrieved from http://scholarworks.wmich.edu/reading_horizons/vol20/iss3/9