Here is the simplified calculation on credit created by banks:
The normal Net Capital Rule is that there should be a 15:1 loan to cash ratio. The crisis occurred as the SEC changed the ratio to 33:1 which double the banks' exposure. With $65 trillion troubled certificates the required cash at hand would be $1.97 trillion or $2 trillion for bad loans. To get back to the normal 15:1 ratio would require another $2 trillion cash (liquidity).
The ratio is really a risk factor. If a bank had deposits of $1000 it can lend out $15,000 credit. Suppose the bank lent out $1000 to 15 borrowers if one fails and files bankruptcy $1000 is lost but the remaining 14 borrowers return the capital of $14,000 plus 5% interest ($700). The bank lost $300.
Suppose we used the 33:1 ratio. With 15 borrowers each receiving $2,200 for a total of $33,000 credit. The bank would have $1000 at hand. If one borrower fails then $2,200 is lost. The remaining 14 borrowers return $30,800 with 5% interest ($1,540). The bank lost $660.
With 15:1 net capital rule, or cash to credit ratio, the was $300 while at 33:1 the loss was $660 so the loss is greater if one of the loan default i.e. firm that got the loan fails. The capital of $1000 is at greater risk with 33:1 cash to credit ratio than at 15:1.