Physics for Dummies: Lux to Lumen

[Corwyn]

Okay. Guys, I want to check the actual output of some flashlights.
I have phone apps that can check the Lux output and sites that can convert Lux to Lumen: http://www.rapidtables.com/calc/light/lux-to-lumen-calculator.htm

Thing is I am mathematically challenged enough not to know what to fill in... how far away should I put the flashlight from the screen, what surface area should I calculate (light circle, screen size or something else), should I take the measurements during daytime or at night...?

[Corwyn]

Physics still boggles me, but I did the redneck test (dark room, flashlight 1 meter away from the screen and counted the Lux asa 1>1 conversion...)
Amazingly this approach worked and even more amazingly, the advertised output was the same as what I got (see 2000 lm for the Ultrafire XML...) ... imagine when my SkyRay King gets here with its retarded 9700 lm 

[dks]

The standard cheap way to compare light output is to shine the light on the ceiling and then put your lightmeter or phone etc on the floor or table. As long as you keep the same distance between the light and the ceiling and the light meter and the ceiling you can compare the light output between torches. By reflecting the beam you avoid measuring just the .middle of the beam which will be more intense especially with zooming lights. With the reflected method a zooming light should give similar results at both zoom extremes as the total output should be the same at both extremes. Blf and CPF had a lot of details on various measuring methods

[jzmtl]

As far as I know they don't interconvert directly, like you can't calculate my weight based on how much pressure I'm exerting on the ground without knowing size of my shoe,  I could be wearing track shoes or snow shoes which would vary the pressure by orders of magnitude.

You need a series of standard known lumen and measure the lux they produce in an enclosed volume, plot them, then compare the lux value of unknown to the line you have to see where they fall.

A cheap way is to have a cardboard box, glue white printer papers inside it, point the flashlight to the top then use your phone next to the flashlight on bottom to measure the reflected light density.

The calculator you linked seem to assume a uniform circle/sphere of light which flashlights don't produce.