how to determine cost basis on a 1099 b form that was left blank from the sale of stock from a spinoff?

You received an IRS form 1099-B that records the sale of stock B.  You received stock B that was spun-off of stock A.  

The issuer of the IRS form 1099-B is not reporting a cost basis for the stock B.  Is this the situation that you are experiencing?

If so, what was your cost basis of stock A?  

Shareholder services of stock would have reported the relative value of the new stock B at the time that the stock was spun off.

As an example, assume your cost basis of stock A was $100 / share.

Also assume that shareholder services reported that one share of stock B was issued for every share of stock A.

Your retain your cost basis.  Before the spin-off, stock A was worth $100 / share and one share was worth $100.

After the spin-off, your one share of stock A is worth $50 and your one share of stock B is worth $50.  The total investment in the two shares is still $100.