Well, acceleration (except for light’s acceleration) is cumulative. So just subtract the acceleration of your jump from the acceleration of your elevator. The difference will be the rate of your own fall.
“It is lucky for rulers that men do not think.” — Adolf Hitler
Yes, you can save yourself if you can jump hard enough, and if there was no elevator ceiling to worry about. But how hard can you jump? Probably hard enough to go 2 or 3 feet straight up, right? But that’s the same speed the elevator would attain in falling that same distance! (remember, gravitational acceleration is constant regardless of weight) So if the elevator fell from somewhere between 1st floor and 2nd floor, you could pretty much cancel the downward speed with a nice big jump. If the elevator started any higher, you’d be in trouble.
Just out of curiosity, when was the last time you heard of an elevator plummeting to the bottom? That plane crash into the Empire State Building is possibly the most recent.
BTW, if you really want to see what the effect would be like…well that is why they put the STOP button in elevators. Same effect and nobody has to die.
For an elevator to plummet, a lot of stuff has to go wrong. Modern elevators have a braking system that uses the tension on the overhead cable to hold the brakes away from the elevator rails. If the tension goes away from a cable break or a motor burnout, the brakes automatically engage the rails and the elevator won’t move.
It would take either damage to the rails, or serious physical damage to the elevator itself to destroy both systems. No doubt something like a B-25 slamming into the elevator shaft could ruin your whole day, but that’s about it. An earthquake, maybe.