Flashlights and headlamps from different brands are constructed very differently which reflects in how efficiently they perform. Therefore we have had to make some assumptions in order to make this calculator.
In the calculation loss of light from the optics have not been taken into account. No flashlight/headlamp optics are efficient enough to direct 100% of the light emitted from the LED into a perfect angled beam. No matter the manufacturer there will always be some loss of light because of the optics.
Some flashlights and headlamps have a focussing system, meaning that the light can be directed from a wide beam into a spot beam. When using the focussing to create a spot beam there will be a higher loss of light than when using the wide beam. This extra loss is not reflected in the calculator.
Another assumption we have made is that the light is evenly distributed over the illuminated area. This would also not be true in most cases, since a LED never distributes the light evenly.
To sum up: In the calculation 100% of the light emitted from the LED is directed perfectly into an angled beam which illuminates an area 100% evenly. This will never be the case, but are assumptions that have had to be made in order for this calculator to work. However, you can still get a pretty precise idea of how many lux a light delivers in a certain area.