I played around with Lexile levels several years ago. What I found was that sentence length is huge, and what is considered a sentence is what is surrounded by periods. So, for example, I typed up a chapter of a Magic Tree House book. MTH is notorious for having sentence fragments everywhere. I used the Lexile analyzer to find the Lexile level of the chapter as written and then I went through and normalized the text, so that the fragments were incorporated into complete sentences. The only words I added were things like "and." Anyway, the grade level of the original text with all the fragments was something like 2.0 and the grade level of the properly written text was something like 4.0--which was what I was expecting.
The Reading Counts levels are pretty good. They show Lexile level, which is sort of a brute force method for calculating reading level, as well as interest level and their own grade level, which is based on Lexile level and other intangibles.
I found all of this helpful when I was trying to track my HGish dyslexic son's reading progress. I found it less helpful with my younger son, who is HG+ and doesn't have dyslexia.
