Mine being on LPG aswell im getting around 18mpg 70% local and 30% motorway(44 ltr = 185 miles) and im a feather foot driver too I like to chill out...... and I have a 58-60 ltr tank. on a motorway run i get about 20mpg and +
I have let it run dry, it will just start to lose power where you then switch it over to petrol (mines not automatic either). oh about the red light it does not mean its near finised ( thats what I had 1st thought it was ) infact depending on your tank size you can get 50+ miles more when the red lights comes on.
A trick I use to find out much is left, other that resetting the trip is when the red light is on and when you come to brake you will notice that the green lights come back on again this a a great indication on how much is left in the tank and you can work out how many more miles you can do, if on the other hand you brake and the light remains red then you know your in your last quarter (again depeding on tank size) and should be able to work out how many more miles are left before filling.
When your on gas your petrol should not be effected, unless the air con uses it ( this I dont know and would be good to find out always wondered if on LPG does the Air Con burn the gas or petrol), As far as I know the petrol on mine is used for a few seconds when starting the car up and then switches over.
I last filled it up and have driven it on petrol now and then (5 miles or so every now and then) to keep the engine lubricated ( not had flashlube fitted yet ) other then than it really has not moved and I do about 40miles a day