Here is the rule. Basically, the qualifying tournaments for your National Ranking average are any class C or above (including Regional Championships or other E,L,R sanctioned tournaments).
A skier may also qualify for Nationals after the Cut Off Date by increasing his Ranking List Average above the Level 8 Cut Off Average for his respective event and division between the Cut Off Date and Nationals; or by attaining a score equal to or greater than the Level 8 Cut Off Average for his respective event and division at the Regional Tournament or at any class C or above event between the Cut-Off Date and the first day of Nationals or by being ranked Level 8 at any point between the Cut-Off Date and the first day of Nationals.
I believe you also receive qualification for a podium finish at this year's Regionals or a podium finish at the prior Nationals.
The top five non-elite-qualified skiers in each age division or overall at the current year’s Regionals or previous year’s Nationals shall qualify for Nationals.
If you are not qualified now, you basically need to raise your average or get at least one score above the level 8 cutoff score. Then, you must attend a Regionals and achieve skiing position to be able to attend Nationals. You also could attend Regionals (assuming you qualify for that, but didn't have a level 8 average or score) and achieve a podium finish or score above the level 8 cutoff to qualify for Nationals.
There is one other way that you could achieve a level 8 average without increasing your own scores. If your current average is close to level 8 and the scores of your division suddenly lower the level 8 cutoff score before the first day of Nationals such that your average exceeds it, you would qualify. Of course, this would depend on a sudden influx of novice skiers posting scores that reduce the level 8 cutoff average score. And, you would still have had to attend Regionals and achieve skiing position.