Improving your cardio probably isn't going to have much of an effect on your "perfectly calm at rest" air consumption all other thigns being equal (e.g. if you lose fat you'll lower your basal metabolic rate which will lower your oxygen requirement). Similar in the way that cardio fitness doesn't really affect your static breath holds in free diving (but affects your dynamic). However, reality is that we are almost never at that basal rate so cardio helps to narrow the gap between an easy relaxed dive and a strenous one. The fitter you are the faster your recovery rate, the less you are affected by x amount of work required, etc.
Average tidal volume is sometihng like 500ml and average breath rate is 12-20/min (just ripped from wikipedia page) so average air consumption at rest is probably 6-10 l/min (SAC of 0.21-0.36) which is a fair bit lower than the corresponding SAC while diving. If you were in a very relaxed meditative state it would no doubt be much lower than that, if you are a 100 lb woman it will be much lower again, etc. so theoretically SAC rates can be far lower than what they tend to be if you are very fit/very calm and do non-strenous dives.
