N.M. Code R. § 13.14.9.37
When only a portion of the land previously insured is being insured by a reissue owner's policy or by a subsequent issue loan policy, or when the land previously insured is only a portion of the land being insured by a reissue owner's policy or by a subsequent issue loan policy, the rates shall be adjusted in proportion to the areas insured in the original policy and the current policy. For example, if the original policy insured one acre and the current policy insures three acres, the reissue rates will apply to one-third of the current value up to the face amount of the original policy; but, if the situation is reversed, the reissue rates will only apply up to one-third of the face amount of the original policy. In no event shall the reissue rates be applied to allow more than one hundred percent of the face amount of the original policy to be used cumulatively in the computation of reissue rates.
N.M. Code R. § 13.14.9.37