If there's not a second cotter pin hole in the spindle and you want to reduce the amount you need to back off after preloading, you could also grind/file/sand the castle nut a little to change the relationship of the castles to the spindle hole. That would probably be simpler than tinkering with the washer thickness.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!