How do we know children are on the correct level reading book?

A Monster Phonics book is at the right level when a child can read the grapheme–phoneme correspondences (GPCs) within it effortlessly. They should also be able to read the decodable words at the front of the book fluently, with minimal or no overt blending, as well as any high-frequency or common exception words. We recommend a 90% level of fluency.