In that hypothetical situation, 160M would only be part of his deduction, he would also reduce other tax burdens due to the charitable deduction also applying.

Also, I would call 0.16 billion a large chunk of 1.8Billion as it would drop that down to 1.64 billion.

Anyway I would not call a basis of 1B out of 1.8B a highly appreciated assets. If he wants to keep a more appreciated asset with a basis of say 1/2 or 1/10th it's current price as a better long term investment you can still do this. Donate the highly appreciated stock, sell some less appreciated stock and use the proceeds to buy the stock you want which now has a much higher basis.

Then if the stock you buy tanks you can at least take a deduction when you sell it.