How do I get the best audio levels when recording a voiceover?

Initially, LumaFusion will automatically default to the input gain specified by iOS for a particular input (built-in microphone, wired microphone, etc).  This will generally produce good results.  Once you adjust the Input Gain setting in the Voiceover Options (...) menu, the Input Gain you set there will be used for any input device, so you may need to manually adjust the Input Gain anytime you change to a different input.  Normally this should not be an issue since you will likely want to use the same input device every time you do a voiceover for consistency.

Also note that 0% Input Gain represents the minimum input gain that the input device allows, and 100% is the maximum input gain that the input device.

Also note that many iOS built-in microphones do not support input gain (you will not see an Input Gain slider in that case).