The trustee for Bernard L. Madoff Investment Securities and the Securities Investor Protection Corp. outlined their legal theory for calculating the amount of customers’ claims and, in the process, revealed details about the world’s largest Ponzi scheme.

In addition to Madoff and Frank DiPascali Jr., the firm’s former chief financial officer, the trustee says the fraud required the “labor of many individuals.” The papers don’t say whether the other Madoff employees were aware of the criminal scheme.

To operate the trading strategy Madoff advertised to his customers would have required more options contracts than were available on the Chicago Board Options Exchange, according to Joseph Looby, one of the trustee’s fraud investigators from FTI Consulting. In October 2002, for example, Looby said that Madoff would have needed 13 times more options than were executed on the Options Exchange for the stock he supposedly was buying.

When the Madoff firm went out of business in December, customers were shown as having $73.1 billion of net equity in their accounts. At the time, the amount deposited but not withdrawn by customers was $20 billion, with the difference representing fictitious profits.

As before, the trustee said that customer profits were “entirely fabricated.”

Last week’s filings by the SIPC and the trustee were made under a litigation schedule established last month for determining the amount of customers’ claims. Customers are to file papers Nov. 13 and Dec. 11. The trustee and the SIPC will file their reply Jan. 15 in advance of a Feb. 2 hearing.

The trustee and SIPC are taking the position that a customer’s claim should equal the difference between the amount of cash deposited and the amount withdrawn. Some customers contend the amount of a claim should take into consideration the time-value of money or equal the balance on the last account statement.

The trustee’s method would be a disadvantage for longtime investors who took little out before the fraud surfaced. On the other hand, the trustee’s theory could help investors who had taken out less than they invested by knocking out claims of customers who had recovered all their cash investments.

Bernard Madoff, the firm’s principal, is serving a 150-year prison sentence following a guilty plea. DiPascali likewise pleaded guilty. The Madoff firm began liquidating in December with the appointment of a trustee under the Securities Investor Protection Act.

Madoff himself went into an involuntary Chapter 7 liquidation in April. His bankruptcy case was consolidated with the firm’s liquidation.