Just as a general statement, when changing from stock gearing, adding two teeth to the rear sprocket and taking one away from the front is a great place to start for shorter tracks. Then you can use the stock sprocket on the front with the +2 rear sprocket for a faster track.
To determine gear ratio, devide the rear sprocket by the front.
Example: 15 front, 45 rear = 3.00 to 1 ratio.
Stock GSXR 17 front, 42 rear = 2.47 to 1 ratio.
1 down 2 up on my GSXR is 16 front, 45 rear. That's a 2.81 ratio, and it's as short as I ever run.
A 17-44 works great for me at Road America, which is a long, fast track.
That's a 2.52 to 1 ratio.
So if you carry your stock number rear sprocket and +1, +2, +3 for the rear, and stock number front and a -1 sprocket for the front, you should have gears for any situation.
My GSXR runs a 17-42 gearset stock, so I carry 16, 17, 42, 43, 44, and 45 sprockets with me to the track.
Lightweight racers generally gear their bikes to top out sixth no matter where they race. For a 600 or larger, try this instead. At a really short, stop and go kind of track, gear to only use the first five speeds in your gearbox. Use all six speeds for Brainerd, Daytona, Road America, ect.