Small cones placed in a kiln sitter are closer to the wall (and the heating elements), so they get hotter and would bend sooner than witness cones places on the kiln shelves. Accordingly, Orton adjusts them slightly to compensate for this. So, for a Cone 05 firing, you would put an 05 Small Cone in the kiln sitter.
Note that many people find that they have to put Cones in the kiln sitter that are 1 Cone hotter than desired to get the correct result on the shelf witness cones. For example, to achieve a witness Cone 06, you might need a Cone 05 (hotter) in the sitter. Or to achieve a witness Cone 5, you might need to use a Cone 6 in the sitter.
