I've been inspired [by this][1] to try using Welder's glass as an ND filter. 

For welders' glass, they use values of "shade" from about 4 up to 11 or so. I know 11 is really dark.  Does anybody know if there is some sort of a formula to use to calculate how many f-stops a shade 10 filter would be, for example.  


  [1]: http://www.thingiverse.com/thing:37967