there has been much debate on pass marks, im not sure if someone has provided a definitive answer on whether it is marked out of how many you did or in total, and what level you have to achieve. in my case i probably did 75-80% of the questions and got most of them right. just work as quickly as you can, don't panic or rush too much at the end.
not sure when you should feel ready, for S1 all you need is to know how the tests work and are confident with the examples and know the motivation paper stuff. there is nothing else to the day really other than a talk from and an ATC.