There is no one formula. This is individual to particular power supplies. Good power supply datasheets will show a graph of efficiency as a function of current. Sometimes you might get a minimum efficiency spec over a current range (assuming fixed voltage supply). Sometimes you only get the efficiency at the maximum output power, which is usually the condition for maximum dissipation in the supply.
The reason there is no one standard formula is that there is no one standard power supply topology, especially when you look at the details. There is much choice of switching frequency, modulation technique (frequency, pulse width, both), synchronous rectification yes/no, type of magnetic core, etc. There are many other tradeoffs possible between size, weight, cost, transient response, output ripple, idle power, efficiency, and more.