minimum lighting level for HPX500 in lux

[FBAS]

Before you say it - I'm going to make my own tests in the next few days, I have a digital light meter and a studio location where I can do the testing - just because I have never done this before I am asking for other results/experiences to compare with.

The background is I'm going to be shooting some nighttime events in Egypt in November. I'm supposed to shoot a documentary-style video of the events and I want to make sure in advance that they hire a gaffer who will at least have a look at the lighting in advance so that the lighting levels will not be too dark. I'd have to tell that guy what I need, though.

I will be shooting with the HPX500 and the Fujinon17x7.6 BRM lens, which is a F1.8-F2.3.

What would be the ideal lighting levels in lux, so that I could shoot at a F2.8 without additional lighting? And what would be the minimal levels so that I'd still have a background when shooting at F1.8 with some on camera fill in the foreground?
Same question goes for 6dB and 9dB gain.

As I said before, I'll do some tests in the next few days and report my results here, I just need to know if I'm horribly wrong because I have near to none experience with the light-meter (just used it to compare lighting levels in studio settings so far)

 

[FBAS]

I did some testing this morning - I used a piece of white paper as reference for 100%, paper was standing on a table in a studio set with very soft and even lighting. As soon as the paper would show 100% in the camera, I considered the lighting to be sufficient for the current setting.
Not a very sophisticated test setting, I know, but this is what I got:

Without gain:
240lx at F1.8
370lx at F2.8

With 6db gain:
150lx at F1.8
230lx at F2.8

The absolute minimum lighting level where I could reach around 80% on the paper with 9dB gain, and around 45-50% at 0dB gain was 50lx. With minimum I mean this would be just enough to not let the background go totally dark when shooting with an on-camera light.

By the way the camera was set to 720/50p, scene file F1 with no changes except News Gamma was on.

What do you think? Are these values plausible?