I'll see your 'rain?', wickett, and raise you a question mark.
Rain??
Keep it flat, unless you want to do some hills. The small incline thing only replicates running up a small incline with a tailwind. True, it can make the numbers on the tready match the speed on the road at a given effort level, but why bother?
If you DO want to bother, the incline needed depends on the speed you run.
<a href="http://www.trinewbies.com/run_treadmill_chart.asp">This link</a> will tell you how.