The purpose of an A-B trust arrangement (also called a «marital and bypass trust combination») is to enable both spouses to use
the applicable estate tax exemption upon their deaths, which shelters more assets from federal estate taxes.
If you have made no taxable gifts, you can estimate the federal estate tax by simply subtracting
the applicable estate tax exemption from your taxable estate, and the resulting taxable value is multiplied by 40 %, the current federal estate tax rate.