Hi,
I had the same problem with my digital camera on a recent skiing holiday.
What I did was to take the polarising filter off the camera and hold it up to the light (in my case a sunny outside window), I then rotated the filter and looked for when the scene was at its darkest, this was at 2 points oposite each other, i.e. 1/4 turn on, 1/4 turn off etc.
I then scratched a small mark onto the rim of the filter and then with the filter set to the full polarising effect another across the rotating section of the filter.
Now when taking photo's in the snow with the filter on I just line up the 2 marks and have full polarising effect.
Hope this makes sense and helps