There's definitely a point at which, even though it may be a slog, you start getting something out of it. IMO you need to be able to get at least the gist (reading something where you already know the story will help there), and then looking up words/asking about grammar points you're not sure of is a side activity. In the past, I've underlined (pencil, in books I own) or kept a notepad and noted down things I wanted to look up.
If you're stopping every second sentence because there's so much unfamiliar grammar/vocab you can't even start to get an idea of what the story is about, you'd probably benefit from putting that particular book to one side and going back to get some more grammar study or to preload on appropriate vocabulary. e.g. if you're reading a courtroom drama and there's a lot of legal terms popping up you don't recognise, go give yourself a crash course in the Japanese legal system and then come back to it.
One thing to consider is reading non-fiction. People always want to jump straight to fiction, but there's plenty of interesting non-fiction and something aimed at native speakers who aren't experts in a given field is ideal for picking up vocab in that area. Lots online, as well. Good search terms here are 入門 (primer), Xの基本 (the basics of...). If you want to read up about Japanese history, food, art, sport, whatever - there's undoubtedly something out there for native speakers who are either new to it or haven't read anything on the subject since they were made to in high school.
These are all comments I make from personal experience, incidentally. I don't mean to discourage anyone from trying, just to encourage you to take a realistic look about your current level and what you expect to get out of this, and how it's going to match up to your own frustration threshold.
Side note: to be honest, if you're still at the stage where you think you can measure your kanji knowledge as a percentage, I suspect real text may come as a bit of a shock to the system.