Proper light manipulation is done with the support hand. Maintaining a firing grip with an indexed trigger finger until you have confirmed your target.
Also, do not go walking around with the light on (weapon mounted or handheld). Proper use is to quickly flash an area that you are interested in, then quickly turn the light off and move; this is why many weapon lights have the momentary 'on' function - and why that switch on the paddle is 'down' for most lights (more ergonomic for RH shooters).
If you have seen a target during your scan, you either engage, or if you are better served by moving (i.e., target is looking at you and is aiming near you), shut off the light, move a step or two, and put the light back on and engage.