If you use a an Etapower l ltr pot and a Standard Trangia and Click stand this would leave around 700ml of space for the alcohol/meths Bottle (assuming a perfect fit). This would boil around 30 litres of water.

http://www.bushwalking.org.au/FAQ/FAQ_Efficiency.htm#Physics

Meths - 18ml to boil 1 ltr of water assuming 80% efficiency

i.e. (700/18)*0.8 = 31.1

Butane/propane mix - 7.54 gms to boil 1 ltr of water assuming 80% efficiency

i.e. (100/7.54)*0.8 = 10.6 (which is near enough to the spec of 12 litres.)