I run 40 - 45 PSI on my size 34 tubulars. If it's muddy I may go down to 35 and have gone as low as 30. I'd be concerned about bottoming out on clinchers though. With tubulars I like to pre ride the course and at least feel it bottom out once or twice then the pressure it right. The step out and snap back in is the deformation of the side wall and again on tubulars this is desired as it actually shocks the impact and keeps your tread on the ground for better traction.