My guess is that it is definitely sub 1.0 gain. Very few everyday household items that are white are actually 1.0 gain. 6ftL isn't unwatchable, I'm sure I'd be able to live with it, that being said, my opinion would be to not bother, there is some light loss with filtering and in your case since your blacks should be reasonable (best case scenario of 0.005 ftL if you adhere to 12ftL), your bulb will eventually dim to 6ish ftL so if you have the ND2 lying around you can see what it will eventually look like. I know preferences will be the determining factor, but reducing blacks without really increasing contrast beyond the point of 10-12ftL can be problematic on certain movies. Better to spend your efforts in tweaking trying to improve shadow detail instead of absolute black levels... here ISF services may come in handy if you can afford it. Otherwise I have no real advice to give on grayscale tracking as I don't have the knowledge or equipment to expand on it. I'm not really sure it is something that can be done without the equipment. (My guess is that it can't)
Cheers...
Duy-Khang Hoang