It assumes you are already at university. You start with 50 points, add or subtract at each question. Your score is your chance of getting a degree. Yes, plus 3. Your score is your percentage chance of getting a degree.
Minimum score is zero.
This is accurate. Some of the negative scores are in fact too small. Statistically, if your parents live in a caravan, that almost excludes a university degree. But in that case your father will probably not have a university degree, or above-average income. To keep the questions simple, they are arranged in terms of cumulative disadvantage.