Burned volume to alcohol percentage In ancient Russia the way to measure "proper" strong spirits was to burn it - if the volume decreased by half than it was considered right one. How can I convert this into modern days alcohol percentage? Also, as a side note, how accurate this conversion would be? I mean which factors besides actual alcohol percentage would affect the process - ambient temperature? humidity? atmospheric pressure? And to which degree: +-1%? 5%?
There are a lot of different factors that provide the alcohols percentage,


  It's not really the liquid burning. It's the vapors that catch fire.
  Higher proof equals more vapor, depending on temperature. source


So burning alcohol is not a guaranteed way of finding out the percentage as whilst cooking wine will 'flame on' despite being low percentage and this is all to do with the vapor of the alcohol. Sure a High proof alcohol will catch fire easier but that will only tell you that it is a high proof alcohol not the percentage in it. A low proof vodka can ignite if a flame is held to it as the change in temperature will make it produce more vapor and it will ignite

(Please don't try and set fire to alcohol if you don't need to it can course spillage that will then cause an out of control fire and we don't want any of you guys to start burning up on us now )