A 81:
That table (on pg. 274) is how you calculate the challenge rating. You use the HP (as modified by AC, as described in "Step 4. Final Challenge Rating") to look up the defensive CR, and you use the damage/round (as modified by attack bonus, as described in that section) to look up the offensive CR, and then you average t defensive and offensive CR together. There is no formula (though you may be able to fit one from the data); you just have to do the table lookup. Also note, of course, that like everything in the DM's purview, these "rules" are only guidelines.

A 82:
By the standard RAW, any humanoid can contract lycanthropy, and kobolds are humanoids. The rules for lycanthropes suggest, as a variant rule, that lycanthropes of non-human origin might retain some of their racial features. Absent this variant rule, nonhuman lycanthropes are still possible; they just have the same stats as human lycanthropes.