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The law that allows state courts to divide military retirement doesn't really dictate any formula that the court has to use. That is left to the state court's discretion.
Legally, she could get 50% of your retirement, even though you weren't married during the entire military service. She could also get 0%, and the court could choose to ignore that your marriage overlapped military service. This is particularly true if the court decides that she has her own retirement funds and so everyone should just keep what they have.
However, if a court were to order division they treat your military retirement like equity in a home and hold that any part earned during the marriage is divided equally. If the court chooses to use that sort of formula, it would look like 8/26 times .50 or about 15%.
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