You should adjust the height of the lamp to get the most even coverage possible, and then play with the dimmer to find the desired light output.
To do this, I strongly recommend you use a PAR meter, or the PPFD Meter apk (which I find more accurate once it's been calibrated than Photone), bearing in mind that the measurements made by a smartphone, even if the camera's CMOS sensor, made up of thousands of photodetectors, is perfectly capable of measuring the quantity of photons received (the opposite would be paradoxical), will not be as accurate as with a dedicated device.