The tax credit shall be allowed as of the taxable year "date of finality" or "semiannual date of finality" as defined in Section 1-101, Tax-Property Article, Annotated Code of Maryland. Such allowance shall be made upon notice being given by the Council to the Director of Finance that by reason of a conveyance, assignment, deed or other instrument recorded among the land records of the County, such real property is affected and servient to a scenic easement which: Is irrevocable by its own terms. Creates a perpetual servitude upon the land therein described which has been qualified as "open space" or "open area" as determined, in writing, by the Maryland-National Capital Park and Planning Commission or the Department of Forests and Parks of the State, or meets the minimum criteria set out in Section 10-241. The tax credit allowed in this Subdivision after having once been determined pursuant to this Section shall remain in full force and effect for all subsequent levies made by the Council.