Best way to obtain a baseline may be to chalk the tires and adjust pressure to optimize contact patch. But someone with a great deal of experience told me good rule of thumb was a pound of air pressure for every 100lbs or vehicle weight would get you in the ballpark. From there you can alter pressures based on handling preference or tire wear.
I ran my Tornado GT40 on a skid pad and measured the tire temperatures on the inside, middle, and outside of the treads. I adjusted the pressure until I got a even distribution. I ended up with 15 psi front and 21 psi rear. My tires are 245/40 ZR17 front and 285/40 ZR17 rear.
I was with an original GT40 1083 at COTA. I think his pressures were actually that or lower. Of course his tires were wider.