Half the voltage should half the current. Should work as a comparison.
Can you beg a light meter off your partner?
You might be able to quantize the difference with readings from it.
If not I have a couple of very old analogue meters we might be able to adapt. Might need some old school mercury batteries though.
I'm interested in adapting a couple of old Maglite flashlight bodies to LED, will PM you for details on the amber ones you have.