How is cost basis for sale of professional corporation stock calculated for tax purposes for partial sale of stock Specifically identify the shares being sold or First In First Out or Average?

You need good financial records so that you know the price you paid for the stock when you bought it. Otherwise, you don't have firm figures to work out the taxable income or to back up the numbers you come up with. Estimates don't really work--maybe if you pay too much in taxes, you won't get dinged. But, since you are in a business to make money, that doesn't make much sense.