Guessing an arbitrary efficiency number will get you within a gross ballpark number, but it shouldn't be relied upon.
Caveat: The simulation results are only as good as the models.
The diode, and presumably the FET (can't tell what that box is, but I'll guess it's a power FET) have resonable models associated with them - a good start. The inductor and capacitors need help and there are models in LTspice's library that attempt to model actual parts. Battery series resistance can be a factor as well as wiring parasitics.
You want to use a realistic efficiency number for the equation given in TI's literature. Fortunately, LTspice has a nifty power graphing and integration tools that givesallows you to calculate the efficiency of the circuit.
- Run the simulation and start recording data when the waveform stabilizes (.tran card, Time to start saving data). Alternately, you can select a steady-state portion of the waveform after the simulation has run.
- Alt+LMB on the power source, V1. This will show a plot of the input power. LMB = left mouse button.
- Alt+LMB on the load resistor, R1. This will show a plot of the load power.
- Ctrl+LMB on the input power graph. This will bring up a window showing the Average input power. Record this number and close the dialog box.
- Ctrl+LMB on the output power graph. This will bring up a window showing the Average output power. Record this number and close the dialog box.
- Efficiency: \$ \eta = {P_{out} \over P_{in}} \$
Beyond this study, to make a useful circuit, you need feedback to regulate the output voltage. There are a number of switching regulator controllers out there that provide a stable output voltage and current limiting.