Our local supplier is one where you take an empty bottle along and swap it for a new, full bottle...along with some money of course

So! How do you tell when you need to do this? I have no gauge on my bottle so I've got no idea how much is left in there, if what I've got will last the trip, or if I have enough to last me for a good few trips...
I don't want to waste money

Should I just bite the bullet, get a new bottle and use a luggage scale to weigh it so I can judge what I use in the future?
Thanks
