I know less than you guys, but I think instead of a percentage threshold, you sould set a min # of GP at the position. To me, I f a dude played 10 years at a posiiton, that is enough to get a decisive view as to where he belongs in the all-time best list for that position.
To take that 10-year guy, and dismiss him as a mere "fifty-percenter" because he then went and played additional 10 years at OF/DH, seems counter-intuitive to me.