Readability
Since both factors may vary independently of each other, the best prediction is gained by devising a formula with makes use of both indices. What this means is that a single score is produced for a text, and that score is looked up on a table or graph. That tells you how difficult the text is in terms of either a) an American school grade level, or b) an artificial scale of 0% to 100%. Either way is effective. What really makes a difference is: