The air pressure in your tires is VERY dependent on the surface, it will vary from course to course. Don't take what I do as a rule of thumb.
As for Matt and his harness, one of the best autoX'ers around here doesn't use one. Instead he acquired a length of cloth (very thick stuff, like a martial artists type belt) put long lengths of velcro on it and wraps it around the seat and his chest. Works great, and is mobile and much much cheaper than a harness.
PS I'm too FAT to move in the seat much, so I don't worry about it.
