My car that has TTI's are 3.5" on the passenger side to the bottom of the flange.
Tires are 25.6" tall, front fender is 24" of the ground and rear is 23".
I could knock a 1/4" off the bottom of the flange for more clearance, I would say this is about as low as you would want the headers for street driving. I would also say that lowering the car any more than this with a stock suspension system is not going to gave you any worthwhile gains in handling.