As explained on the Stock Rights pageof this website, you may (but are not required to) allocate your cost basis to stock rights that you receive that represent less than 15% of the combined value of the rights plus original stock. If the rights represent more than 15% of the combined value, you must allocate your cost basis; it is not optional.
Optional cost basis allocation is elected by attaching a statement to your tax return and a statement is required for major holders.
Click on the image to the right to access the Stock Rights Calculator for easy computation of your cost basis allocation for stock rights issued by companies you own.
Data for recent stock rights issuances such as Barclays plc, Lloyds Banking