I'll take a stab at this one Spitfire.  Just be aware this is my own opinion, and not legal advice.

As far as Revenue Canada is concerned, your ACB is .06 in the scenario you propose, so a gain of .14  Revenue Canada will always go with what maximizes their tax in less than obvious situations.

However, if I were in this situation, I would follow the documents I have from my bank, and show an ACB of .12 and a gain of .08

That's based on what Revenue Canada will be able to discover if they ask for some proof.  If they did, you would show them the document issued from your bank.  

Honestly, I believe this is a grey area, like many other tax related questions.  Not everything is black and white in tax law, and so there are various interpretations made.  A close friend of mine is a CA who deals with Revenue Canada often, and frequently is asking for their interpretation of various tax situations.  Their responses vary widely, depending on who is responding.

So, the short of it is, I would go with the bank documents.  If later they disagree with that, no harm done, as long as you are acting in good faith.   At that point, they will just do a reassessment and you'd have to pay more.