For starters it depends on what tire you run.

I run the stock Michelin Sport

I started at 39 rear and 41 front per Wes M. I had good tire wear in the fornt. This weedend I went to 39 all around, car drove better, but I did see more wear on the front tires.

Tire pressure also depends on your driving style and how you want the car to feel.

Try your first run at 40 all around, Second run see how you like the feel and look at the tires (use chalk) because the air will expand due to heat, you will find that when you ck the pressures all four tires will be a little off, you will also know what tire is workin the hardest.

Before the third run, bleed the tires to a pressure you want to set for your last speed runs. I would not go any lower than 38 based on what I have seen.

Let me know how it works for you!