your setting options are the same as mine ( a G3)
I've recently captured some decent shots. I tried setting to a 15 sec exposure and guessing. but that wasn't very succesfull. mostly because the lightning that night was sporatic. I have had some succes by shooting on continuous mode until the buffer is full. you'll take a lot of pics, but if they're pics of a black sky they won't take much space in memory. The last time I did this I fit about 300 shots on a 128mb card.
If you are interested, you can see my lightning pics here