Does the real-world MacBook Pro battery life scale linearly with it's capacity?
First of all a note, coconutBattery is a tool which helps you indicating a battery problem. It's measurements are not 100% accurate...

If you really want to know the health of your battery, I suggest you take a look at System Profiler (Applications &lt; Utilities &lt; Sytem Profiler) and open the 'Power' bar. Here you see information comming directly from Mac OS. It could be the same as coconutBattery, depending on what version you use, but this is the most reliable information about your battery...

You have Charge Information and Health Information  

  

And to answer your question, I don't think their is a strict correlation between the lifetime of a battery and the number of mAh left (at least none that I found of). There's been a lot of writing about batteries, their lifespan and their health. But it depends on so much external factors, you can't make a decent prediction of it.
As your Mac ages, you battery's health will go down. Learn to live with it :)