The closest thing to a widely-known scientific measure of grade-level in terms of rating reading material is probably the Flesch–Kincaid readability test. However, it has been noted that whether the student is interested in the material has a huge impact on whether they can read it.
Another approach to determining whether reading material is appropriate for the child is the Zone of Proximal Development (ZPD). A ZPD rule of thumb is that a child will learn the most from reading material where they recognize about 90% of the words. Gifted children rarely encounter such texts in school, but usually manage to learn to read by reading what they enjoy.
In summary, don't look for reading levels to be mathematically precise. It's not a particularly helpful quest.
