I didn't say anything about your simplified 20% assumption, because with a fully valued company, it's true the sp would basically just go up 25% if you leave the market cap constant and reduce the os by 20%. However, the impact of buying that 20% would need to be calculated using the trading float rather than the entire os, since the shares which are not for sale couldn't be bought. Of course, the time variable would also have to be considered. 150 million in one day is a whole lot different animal than spread over a year...
Suffice it to say I'm in the camp that when the buyback kicks in in earnest, we'll know it, and there will be no doubts.