Having chatted at long lengths once about the recruitment process with HR (after I got accepted), I was informed that the test that most people fail with at stage one is the basic checking. When I say most, she was giving me numbers of like 70 to 80% of stage 1 candidates.
You must pass all tests indiviudally to pass, it's not a combined score. There is no pass mark published, the rumours of 70% you are probably hearing are the pass marks used at the college. I don't think on my basic checking test I got through 70% of the questions. But I'm pretty sure I got the ones I did all right.
So it's all about accuracy, not quantity. You can find tests like it on the internet, or you can get a friend or family member to make up a mock test for it? It's not too hard just get them to make up a 4, 5 or 6 group of letters or digits (I don't think it was mixed, either just all numbers or all letters), and then a selection of 5 very similar ones to pick from, one of which is correct.
Good luck.