If you are having problems with ign it is unlikly going to be to do with timing it's very rare for it to go out of time, plus L.P.G runs fine on standard ign timing, it's more likly a coil, lead, or cap problem,
As a rough and i do mean rough guild i'd set points gap at about a 1 1/2MM - 2MM or tip of a small screwdriver,
If it's not running rough on petrol your gas system might of gone out of tune, and need the evaporator adjusting?
hope this helps
:)