beneficiary designation form

[ERISA25]

Has anyone come across any private letter rulings, or are there any cases, or regulations discussing on what basis an administrator may reject a beneficiary designation form? For example, if the participant indicated that both primary beneficiaries should receive 100% of the benefit or if the total designation exceed 100% on the benef designation form. My thought is that it is ambiguous and the plan document would control. See Metropolitan Life Insurance Company v. Parker, 436 F.2d 1109 (9th Cir. 2006).