Hi again BACK_TO_GB,
When you will apply for your first FLR, the home office will issue 3 years visa starting from the date of issue. Lets say you sent your FLR application on 1st of August 2009. The HO processed your application on 19th of August and issued visa on 20th of August 2009. This visa will have validity from 20th of August 2009 to 19th of August 2012. Which means you lost extra 11 days for your ILR application at the end of 5 years period.
Now lets look into another example which probably you want to follow.
You sent your FLR application on 28th of August 2009(2 days before your visa expiry, you are allowed to do that as long as it reaches HO before 1st September 2009). Lets say they processed your application on 15th september and stamped a visa valid from 16th of September 2009 to 15 of september 2012. This means you have gained extra 15 days to file your ILR
I hope you are able to understand what I am trying to explain.
About the duration you can stay outside UK, I dont know much about that as I havent done enough research on that area. Hope somebody else can shed some light on it.
Best of luck with your future in UK.
P.S. I am also going to UK on 24th of August. When r u planning to arrive?