Sparkling wine and champagne do not necessarily contain the same amount of Co2. I've had sparkling wine that has had a very low volume, especially sparkling reds. Champagne has pretty high pressure, probably about 5-6 volumes.
Personally I don't enjoy drinking something as gassy as champange and I would probably go for about 2.5 volums in my mead.
All information in the post is basically made up and may be completely inaccurate.