This is a difficult question to answer without knowing your weight, riding style, etc. With that said, I would start around 32 psi and go from there. Make sure that you are using a good tire gauge. The gauge on your pump measures the pressure in the hose, not the tire, so it's not accurate enough for the low pressures of CX.
Typically you want the lowest pressure that works for the course. You don't want to be folding the sidewalls in corners unless it's incredibly muddy, in which case you're going pretty slow in corners anyways usually.
Typically with tubulars you want to feel the rim once a lap, or close to it.
In general I'd say find a pressure and then over subsequent laps at practice let 1psi out each time. You can feel a real difference in the corners. And at some point the tire will get too squishy for the speed you're taking around corners and you've learned your limits.
Both Jeremy Powers and Shawn Adamson have told me that it takes awhile to learn to corner on tubulars and suggested just taking a lot of time to corner on them and play with the pressure. So far that's working out well for me.