I accidentally bought a 26.8 seatpost instead of a 28.6. I could only find a 25-26.8 shim, which was the right thickness. It worked perfectly, although it stays with the seatpost rather than staying in the seattube when the seatpost is loosened. So, probably any shim of the right thickness would be better than one that is set for one of the correct diameters but the wrong thickness.