I had an 1120-S I wanted to get out the door so I filled out a 7203 using the fillable pdf on the IRS website.  After all, the instructions say that learning about the law and the form should take only 15 minutes.  

It took me somewhat longer than that to get past Line 1, which asks for the basis of the shareholder's stock.  To get the answer I wanted, I had to use the balance sheet stock basis plus retained earnings.  This might be wrong, but no one is going to look at it anyway and there's no penalty as long as the shareholder can still take the $2,000 loss since he has $2,500 in capital stock, $55,000 in retained earnings and $4,000 in loan to corporation.  It just matters which account gets reduced.