What do you do when you purchase a small slice of a company?
If you've purchased less than 20% of a company, you can stick the purchase on your books using the fair value method.
It's actually quite simple. You use marked to market systems to put the purchase on your balance sheet at fair value.
As a result, your net income will rise and fall due to unrealized gains and losses from changes to the share price as well any dividend distributed.
Calculating is simple: Just take the original price, add the gain (or subtract the loss) in stock price, and you're done!