(a) Unless otherwise expressly provided by a will or other controlling instrument, under which a gift is made to or for the benefit of the surviving spouse of a decedent which qualifies for an estate tax marital deduction under the tax law of the United States and the amount or size of the gift is defined by the terms of the will or other controlling instrument in terms of the maximum marital deduction allowable under the tax law, the definitions do not constitute a direction by the decedent to the fiduciary to exercise an election respecting the deduction of estate administration expenses or the determination of the estate tax valuation date, which the fiduciary may have under the tax law, only in a manner as will result in a larger allowable estate tax marital deduction than if the contrary election had been made.