Question: how do you know how much gas is left in the bottle?
Answer: Fill up a jug or bottle with warm tap water.
Pour it down the side of the gas bottle.
Wait 5 seconds.
Using your hands, feel for a temperature change on the bottle.
The part that's cold to touch will indicate how much gas is left. If it's cold near the top – you're ok. If it's warm the whole way down…