That is when I stumbled upon this handy tool:
The Fernandez-Huerta Readability Index Calculator
With this "calculator" you are able to copy and paste any text that you find and it will tell you how difficult or easy it would be for a Spanish-speaking person to read and understand. I have found this very useful when screening websites or articles for use within my classroom.
While each group of students is unique, I would recommend the following as a starting point for middle/high school Spanish courses.
Spanish I Index 80-100
Spanish II Index 70-90
Spanish III Index 60-80
Spanish IV Index 50-70
Spanish V Index 40-60
AP Spanish Index 30-50