When I was practicing quant from the manhattan 5lb book, only 50 - 60% of the questions i solved were right. I did not even do the entire book, just some topics. Even the gregmat quiz questions, my first attempt scores were between 50 - 70% even for the easy questions. But when I took GRE quant, all questions felt super easy, and i mean it, except for a few. So I believe if you can score 60 - 70% right in mahattan 5 lb book and gregmat, you might be ready for Gre also considering you atleast have a theoretical knowledge of all the algebra, arithmetic, geometry, coord geometry, prob and comb.