The LabFlicker instrument and software will automatically tell you when the sensor is at a suitable distance from the light source. If you move the flicker meter as requested by the software any level of lux can be measured. The actual sensitivity range of the sensor itself is 1,200-11,000 lux. Measuring this way gives the best signal to noise ratio meaning your measurement will always be very accurate.
If you measure at e.g. at a distance of 300 cm (3 meters) with a normal luminaire of 800 lm you will get a bad signal to noise ratio and a less accurate result. This goes for similar instruments from other suppliers as well.