Nobody needs
only 650+ or 700+ questions, because we all have to answer some questions that are below that level in order to get to the higher levels in the first place.
Further, if you want your score to be a 700-type score, then two things need to happen: you have to be able to answer 600 to 650-type problems mostly correctly, of course, but you
also need to be able to answer them in less time than average. These questions will become the easier / lower-level questions for someone who's scoring at a high level, so they need to be fast, effective, no or very few careless mistakes, etc. That takes practice and continued study!
In general, the higher the question number (in OG), the harder the question. This isn't 100% universally true in a straight line for all questions, but that is the general trend through the OG. So, go ahead and concentrate more on higher-numbered questions if your score has already put you near that range, but don't neglect those medium-level ones!