tada - in 2013, RVX didn't give a value of Zenith shares rather they gave a percentage of the estimated market values of the two company's shares at the time. That percentage was 9.1% which would make the cost 9.1% of whatever the cost of the RVX shares that were split was. 90.9% of that cost belongs to those RVX shares. You are correct that this is not a fixed amount rather Resverlogix's best estimate at the time. You should use something that CRA will accept. I would think though, that the company's estimate of market value for the shares of each company at that time would be the one that CRA might consider the most reliable. Up to each to try what they think will work.