A former spouse gets nothing unless provided in a QDRO. Unless the participant dies before the plan determines the QDRO, the QDRO can invade the "rights" of the second spouse. The formula you describe is conventional and is not regarded as overreaching with regard to the second spouse. This a bit complicated, and one could quarrel with the articulation, but you, as subsequent spouse, do not "vest" with repect to a QDRO until the participant dies. As a matter of value judgment on my part, you have nothing to complain about if the former spouse is awarded half of the benefit accrued during the time of marriage to the first spouse, including the survivor benefit that goes with that half.
To get closer to your question, the terms of the QDRO determine the disposition of the survivor benefits. As subsequent spouse and designated beneficiary, you have all of the survivor benefits that are left over after the QDRO alternate payee's survivor benefit (as expressed in the QDRO) is honored. You may have some portion and the former spouse may also have a portion.