Years ago, we had a few annual balance forward plans that the plan administrator/trustee set some amount that if the distribution was more, then either wait till the next valuation, do a partial distribution, or do a special valuation. So if the distribution was more than, say 10% of total plan assets, one of the 3 options would be used... or if the distribution was less than, say 10% of total plan assets, the distribution was paid based on the last valuation. The percentage of total plan assets, and the options to be used, were decided by the plan administrator and applied consistently for the plan.