Bank Capital?

So, let's run a bank. We accept money from depositors and buyers of bank stock. We lend this money at interest. Question, how much money can we lend? All of it? Not hardly, someone will make a withdrawal and we have to have cash in the till to pay out. Sometimes a loan won't be paid back, and a decent bank can't fail just 'cause one lowlife stiffs them on a loan. So, how much money do we keep on hand as protection against rainy days?
Experience and federal law suggest a bank ought to keep about 10% on hand as "capital" or "capital reserves". The commercial banks adhered to this. The "investment banks" aka stock brokerage houses, ignored this rule, went down as low as 3%, and are now all dead. RIP Drexel Burnham Lambert, Merrill Lynch, Lehman Bros, Bear Stearns, and company.
Now keeping plain old cash in the vault is secure, but that's a lot of cash that isn't earning interest. Can we count ultra safe securities, T-bills say, as "capital reserves"? Well OK. How about blue chip stocks and bonds? Perhaps. How about riskier stuff like mortgage backed securities? Well, we allowed that to our sorrow.
Next question, after we demand that banks keep a certain amount of capitol on hand, and after we allow things other than cash to serve as "capitol", how do we count said capitol? Do we count the stocks and bonds and riskier stuff at purchase price or at current market value? Speaking as a depositor, I say count every thing at current market value. The purpose of the capitol is to pay off depositors and cover bad loans. To do any good, the capitol must be converted into cash, the stocks and bonds sold. "Capitol" that cannot be sold at all, or only sold for very little money is worth little to nothing when it comes to paying off the bank's obligations.
Hence the "mark-to-market" rule which banks hate. Under "mark-to-market" the bank must count as capital only the market value of securites. The rule was put in after the Enron scam which involved carrying worthless stuff on the books at purchase price.