Cool! Do you do white-balancing with the ND inplace? I try to keep up by constantly sampling the light during the time I am shooting. I use a home-made light diffuser over the lens and sample the white balance. It prefer to do it this way then trying to adjust the white balance late in post.
The funnest home-spun created ND filter I ever used and still use is using a #10 welding glass. I've no idea the level ND it would be, but it usually requires a few minutes long exposure. Output would be tinted green and most color information would be lost, so best to use when wanting the output as a B&W.
I Have my WB to automatic, besides, with this ND filter I can't see anything through the viewfinder! I really don't care about this, I have the RAW power ; ) I really like the quality of this filter, doesn't affect the color.