I'm thinking it this way:
1. Table has to be as close as possible when it comes to points.
2. Consider 2 teams already doomed to oblivion i.e. 0 points. We are considering the teams in 17th and 18th with as many points they can get without ensuring survival.
3. As a result of #2, we have to consider 4 wins (over the 2 bottom teams) each for the rest of the 18 teams plus a balance of 1 win/ 1 loss .
That would mean a safety mark of around 63 pts (4 wins over bottom 2, 17 losses, 17 wins).
Am I making sense? Synapses dulled by afternoon stupor so forgive me if I create more confusion.
Edit: Revised.