I thinks it's fair to say that if you end up on more points than the team in 3rd place that there is a good chance you could be in the top two.
Regardless of number of points.
I'm with Tricky Dicky on this. If you finish 7 points ahead of third, six of those points weren't needed
In the last six seasons, the number of points achieved by the third placed team has been 79 (04/05), 76 (05/06), 83 (06/07), 80 (07/08), 87 (08/09) and 85 (09/10). The average number of points over those six seasons is 81.66 as posted by Seagulls Till I Die. However, that average hides an upward trend in the points achieved. Taking two seasons at a time, the average for seasons 04/5 and 05/6 is 77.5, for seasons 06/7 and 07/8 it is 83.75 and for seasons 08/9 and 09/10 the average is 86 points. This suggests that at least 8 wins and a draw are necessary to ensure automatic promotion.
To simplify things, I reckon stick with the 2 points per game theory. If our rivals each managed this in the last 13-15 games then BMuff would have 84, with SCC and Hudds both on 82. I really can't see them getting any more than this so 83 points is probably the finishing line as far as promotion is concerned.
For the record, the points total of the team in 2nd for the past 6 seasons was; 86, 89, 82, 85, 79, 86.
So an average of 84.5.
So STID's idea of another 20 points (meaning we'll be on 82), would (according to these pointless sums) mean we finish third.
But it's the second placed team you should be working out. 3rd place only gives you a play off position.