$60b, if that is the amount the government will conclude is the reasonable requirement, will be raised based on the value of the company. The government isn't going to value the business at a particular amount in order to satisfy shareholders. They have to value a business reasonably enough that future investors will want purchase the new issues. Therefore, a number simply picked put off thin air, as you suggest, isn't going to happen.
The general rule for valuing businesses is based on the theory that value is equal to all future expected cash flows discounted back to the present. In the case of Fannie Mae, the answer is already supplied. Over the last 4+ years, the government has confiscated nearly all of their annual earnings while leaving some available for their cash and investment accounts. That is the amount they'll use to base the value of the company on. The rest, is simple math that I've gone over in detail in the past. The value of the business is extremely predictable and easy to ascertain. What it won't involve is simply picking a number out of thin air.
$255b has been paid back out of $187.5b that has been injected, leaving $67.5b as the amount of surplus recieved by Treasury. In that time period (8 years), equity has fallen from $44b to $6b which means value has been eroding because of the constraints placed on the business. However, they've been able to put away a little cash which equals $25.2b and the mark-to-market value of their securities equals $48.9b. Because $9.4b of that cash is already committed to short term interest obligations and because the majority of their $6b of equity comes from deferred tax assets which will be marked down by $13b in the next few months, the cash they have recorded on the books is not considered "excess cash" and cannot be used towards valuation.
It's simple math.
The GSE's haven't produced $255b. $187b of that amount was given to them by the Treasury which they simply gave back. What they've produced is the difference between the two amounts. Over 8 years, it equals $8.5 per year. Hardly enough to make a dent on a $100b per year expense.