(b) Notwithstanding paragraph (a) of this subdivision, no less than 10 months after the effective date of this section and upon the promulgation of a rule by the department stating that the federal department of housing and urban development has provided at least one performance characteristic sheet or other sufficient written technical guidance approving a commercially available x-ray fluorescence analyzer tested at the level of 0.5 milligrams of lead per square centimeter, "lead-based paint" shall mean paint or other similar surface coating material containing 0.5 milligrams of lead per square centimeter or greater, as determined by laboratory analysis, or by an x-ray fluorescence analyzer. X-ray fluorescence readings shall be classified as positive or negative in accordance with such performance characteristic sheet or other guidance. If laboratory analysis is used to determine lead content, results shall be reported in milligrams of lead per square centimeter. Where the surface area of a paint chip sample cannot be accurately measured or if an accurately measured paint chip sample cannot be removed, a laboratory analysis may be reported in percent by weight. In such case, lead-based paint shall mean any paint or other similar surface-coating material containing more than 0.25 percent of metallic lead, based on the non-volatile content of the paint or other similar surface-coating material.