The acrylic lid attenuates the meter reading from 99 to 89 lux with it sitting on my desk and the overhead CFL light lighting the room. That would be a 10% loss due to the lid, which would just change the conversion factor from 60.6 to 67. Not a problem, that I can see.
There is a tiny "instruction manual" with the luxmeter, which has this chart on it:
Clearly this is not consistent with a true PAR meter, but calibration against a PAR meter for specific light sources should take care of this easily enough.