Difficult to answer that, rider1520. Vaporization rates are affected by a number of things and practice sometimes overwhelms gas theory (Charles Law). Temperature and the amount of liquid LPG surface have an effect, so often a tank that is 1/2-5/8 full seems optimum, even though theory says it should make no difference. Remember that a "full" LPG tank is still only 80% liquid, so there is always some vapor space. As the liquid level drops toward 1/2, you get a lot more vapor volume and, in a horizontal tank, the surface that is evaporating gets larger too. However, the gas pressure stays the same (in theory, anyway) as long as the rate of evaporation equals or exceeds the rate that gas vapor is drawn off. The gas vapor draw is at very low pressure, about 0.5 psi, so even with the furnace gulping it, the tank doesn't need to generate a lot of vapor.
The rush of gas vapor through the regulator also causes its temperature to drop, actually below ambient, so a regulator can "freeze" before anything else, especially if there was any moisture trapped in the air inside. Again, exactly what happens in any specific piece of hardware may be somewhat at odds with the general theory of how such things behave. Trying to explain the nuances with theory gets tedious. The most practical approach is to insulate or warm both tank and regulator a bit and see if it has any helpful effect.
)
