With LPG it is better to have spark plugs a little colder.
Why? Because LPG burns a little bit hotter than petrol (gasoline).
But...
Beware of LPG in our cars, because the valve seats are not suitable for LPG for any longer period of time! This unfortunately being shared with most Japanese-maker cars. If the Mirage were suitable for LPG in the long run, I would have converted it to LPG from day one! Many of my previous cars ran on LPG.
The valve seats on LPG will slowly burn away causing the valve play to increase. If the car is driven hard, it happens really quick, perhaps before 50000km. If this has happened already, after valve adjustment you might consider to run it on normal fuel instead for the rest of the car's already shortened life.