Only saw your question now. haha. I just did a test of the flash during taking a photo in the same light box. I did it a few times and I was able to get around 80-95 lux for that instant. (compared to only around 23-25lux for flashlight torch mode) That's nearly 4 times the output. To produce 4 times its output, it might need 6-8 times the power due to loss in efficiency. (eg. in high power LEDs, 1W might produce 100 lumens but it requires around 3W to produce 200 lumens) So your calculations actually look pretty close. =)