# spinoff entity value / adjusted close of a spinoff

When company $$A$$ spins off company $$B$$ (i.e. $$A = A'+B$$), how do we know exactly the adjustment factor of $$A'$$ and $$B$$ before trading

Note I am not asking to value the new companies. That's a whole different topic. I am just asking for the “adjustment price" of a corporate action. For example if $$A$$ closes at $$\\\100.0$$, it pays $$\\\5.0$$ dividend, then the adjusted close of $$A$$ is $$\\\95.0$$. The adjustment factor on bloomberg terminal would be 0.95. Before market open, suppose nothing happens, we know the "fair value" of $$A$$ is about $$\\\95$$ since it loses $$\\\5.0$$ of value due to dividend.

But when yesterday AIV spinned off AIRC, how exactly do we know how much value is in AIV and how much is in AIRC? Turned out AIV is valued at around $$\\\5.47$$ so it spinned off almost 90% of its value.

Supposedly the price for AIRC is tracked by "AIRC WI" but that ticker had no trading on NYSE the day before as far as I know. It was also incredibly hard to look up the prices of "AIRC WI" the day after when the spinoff really happened (i had to dig through raw market data). Is there a better data source?