Ampere level through switch must go higher (by 10 to 50 percent) than desired Ampere output.
The switch-On time is based on the inductive time constant L/R.
...where R is impedance through the current loop at that moment. You must leave the switch On for as much time as it takes for Amperes to rise through the inductor.
Notice a reservoir capacitor at the output offers little impedance... However it opposes current in the sense that its voltage charge opposes voltage coming from the system.
Since you state the load is light, you don't need much On-time. Your reservoir capacitor discharges little during idle times of the cycle. This makes it hard to calculate On-time.