Calor always fill their bottles by weight (as do most other fillers) so the empty bottle weight must be known. It should be tramped on the disk or some other part of the bottle under the title of 'TARE'
Don't know if they still do it but Calor used to show the weight of gas in Metric and the tare weight of the cylinder in Imperial on the aluminium collar on the top of the cylinder. Haven't noticed lately if that's still the case and it's too late to go mooching in the front locker.
the weight on the disc is written in pounds and ounces , i weighed mine the other day the total weight was 25 Lb , the metal disc said 18 Lb so that left 7Lb of gas divide 7 by 2.2 gives me 3.18 Kgs of gas left