Take the profit margin of the current loadfactor, that is the break even load factor (for example: 15% profit, 80% load factor: .80/1.15 = 70% break even load factor). Because if you had 15% less income with the same amount of cost you would not have a profit, so break-even.

But if you are smarter than a fifth grader you will see it doesn't work that way. To get 15% less people to fly you would have to raise the prices, and lighter airplanes use less fuel, and less pax means less bags, so less luggage haulers needed, and less people to answer the phone at customer service (Oh, wait....). How much the price has to change to affect loads depends heavily on what peer carriers do to match. Because of this BE-LF is not a very usefull statistic.

Pretty much the only way to know the break even load factor is to not have profit (or loss), and look at the load factor. I would prefer the company I work for not to know.......

Independence air needed 120 pax on the A320, and they needed 60 pax to break even on the CRJ 200. Took them too long to get enough 320s (or something like that, bit fuzzy after 2 decades)

I personally think the more usefull metric is how much money per pax do you make, compared to the competition. NK has been at -$20 for a long time. As long as you make a Buzzball/pax you are golden.