I was wondering if someone could explain to me how Sudden Death Executes are calculated with the bonus rage.
I am assuming in my explanation, from what you said, that you are specced into Improved Execute.
The execute begins with a base rage cost of 10. As the tooltip says, it uses up to 30 rage total (as opposed to 30 extra rage). So, you have 30 rage used, a base cost of 10, and 20 extra rage being converted into damage. Now, with the glyph, it adds the equivalent bonus damage of 10 rage. So you have, in the end, a 30 total rage cost, a base cost of 10 rage, and bonus damage from 30 rage. Without Improved Execute, this becomes 30 rage total, a base cost of 15, and bonus damage from 25 rage.