Consider this example:

Siemens is going to have a spin-off (next Monday). If I owned, say, 10,000 shares of Siemens bought at an average price of 80 euros, my cost would be 800,000 euros. In the spin-off, for every 10 shares of Siemens, I will get 1 share of the company OSRAM Licht AG. So, I would get 1,000 shares of OSRAM.

How can I correctly split my original costs between the shares I'll have in both companies?

I was thinking perhaps this way:

On July 5th, 2013 on the Frankfurt Stock Exchange, the closing price for Siemens was 78 euros. Assuming the price on Monday, July 8th, drops to 75 euros (ex-spin-off price), then the market price drop from 78 to 75 euros can be assumed "adjusted" due to the spin-off.

Should I multiply 3 euros by 1,000 OSRAM Licht AG shares, and deduct 3,000 euros from 800,000 euros to re-value my Siemens shares, to 797,000 euros? And value the cost of my OSRAM Licht AG shares at 3,000 euros?

Or, is calculating the cost basis for shares due to a spin-off not as simple as this, and other information is necessary? If so, what do I need to know?