I find in my 2013 AT-X, as soon as the needle goes to around the 1/4 mark, it takes around 40-43L to fill back up to full.
(Edit : Just re-read your post and you have the 2015 AT-X - presumably with the updated instrument cluster - I have the 50-series MkI AT-X)
I did a long-ish trip, around 550kms and the needle, was at 1/4, I filled up 40L which is pretty good economy.
I used to run my old 1994 Honda Accord to dry every tank, and the fuel pump eventually gave in. Since then, I've owned a 2003 Holden Commodore and 2008 Ford Falcon both on LPG/Dual and never have let the tank go below 1/4 - I'm sticking with that.