For best range, the goal is to fly at the highest altitude that you can sustain best range power. That's about 18K at high weights. As we lighten with fuel burn over the course of an 11 hour flight, we climb about 2,000 feet every 2 hours. The airplane is certified to 31K, but the pressurization system can only maintain a 10,000 foot cabin altitude to 25K, so FL 240 is our highest practical westbound flight level. The paragraph above assumes no wind or tailwind.
Headwinds require a different profile.
For example, If the headwind component at FL 240 was 10 knots greater than at FL180, best range is achieved by staying down at FL 180. We basically need all four. The standard 121.5 MHz aircraft ELT has some limitations. Generally, two passes of a polar orbiting satellite are required to localize your beacon. The satellite must have both the activated emergency beacon and a ground station simultaneously in view.