There must be a policy in place and be followed consistently. If the portfolio value falls below the minimum due to withdraw/market impact, the policy would determine when to remove the portolio from the composite (for example, if the portolio is under the minimum in 2 consecutive quarters, remove it on the first day of the 3rd quarter). However, it is advisable to examine the impact of removal of a large number of portolios during financial crisis when most portolios fall below the min level. Minimum level can be revised and reset prospectively (not retrospectively) in such events.