It can't be calculated because there is no way to measure the exchange rate of the moisture-laden air above the surface of the fuel. The exchange rate of air, the available surface area, the rate of methanol evaporation and a host of other factors makes this calculation essentially impossible. Having said this, I would seriously doubt that there would be enough absorbed moisture to have any serious effect on the fuel in 30 minutes! Perhaps if you were actively bubbling the air through the fuel at several litres per minute you might have a problem!

Ross