Last year 8th place had 27, but had Parramatta not lost points, they would have run 8th on 30.
Every other year since 2011, 8th place has had 28. Some years teams on 28 have missed out.
In 2010 the Storm lost all their points, but would have run 5th on the wins they had. The 8th team would have had 30.
Leaving aside teams losing points, what tends to push up the points needed for 8th place is a team (or more than one) that wins very few games, like the Knights last year. Conversely a tear away leader that loses very few games tends to lower the bar for 8th.
At the halfway point 16 points is 7th spot this year and
It's almost impossible to gauge what 8th will need from what 8th has now, because some teams have had a bye and others haven't, but going on history if you reach 28 with a good positive points difference, you'd be unfortunate to miss out.