I do the same as Gnashings. It works. You can be sufficiently accurate to get half polarised rather than just "on" or "off". The more marks you have round the edge the better and if there aren't enough then add some more with differently coloured nail varnishes or whatever on the mount. Heliopan ( I think) make polarisers with lots of numbers round the edge for this very purpose.

Word of warning with the Xpan though- especially with the wider lenses differential polarisation can spoil your photographs.