The pressure is only significant when the tank is almost empty (like about to run out completely), and is dependent on the temperature. That's because the gas is stored as a liquid with a vapour space above it. The pressure in the vapour space is dependent on the temperature of the gas, not on the volume of the liquid.
The best way is to weigh the cylinder and subtract the empty weight. According to Calor's website, the tare weight is printed on each cylinder.