Alkaline "% full" from voltage?
There's usually a V vs. Ah curve in the datasheet for the model of the battery, which you chose.  If the battery comes from a reputable manufacturer, the datasheets may be available on the web.

Example:  from here.  In addition to the useful link, which Brian had posted.

You can also make your own measurements and generate/verify the curves.  You would characterize the battery yourself.

Keep in mind that the curve is dependent on temperature and rate of discharge.  Essentially, there is a family of these nonlinear curves.  It's usually not practical to try to account for these parameters.  As a result, the error can easily be as large as 20%.  Battery voltage measurement could be an adequate approach if all that's needed is to make a threshold comparison and blink a "battery low" indicator.