What makes you think you cant use all the gas in the Calor cylinders?
In common with all types of vapour take off LPG bottles, the content will be delivered provided there is sufficient pressure in the cylinder. The pressure is dependent on the temperature of the cylinder, and is different for Butane and Propane.
Whilst there is any liquefied gas left in the cylinder, and it is warm enough, it will continue to vapourise and regenerate bottle pressure. When the liquefied content has been exhausted, the vapour pressure will drop as the vapour is used. It will stop delivering vapour when the cylinder pressure matches the outlet pressure.
In normal use you will never completely empty an LPG cylinder of gas vapour, there will always be cylinder's volume of vapour remaining, but as a fraction of the original content it will be no more than about 0.15%.