There really is no hard-fast rule with DH tire pressures. A lot of it is personal preference and how you like your bike to feel and how you have the suspension set up. Experience plays a big role as well. An experienced (or simply skilled) rider tends to be smoother and won't flat as much.
With a tire that narrow, I would run a fairly high pressure. The one nice thig I discovered about UST tires is you can run high (for DH) pressures while still maintaining good grip and feel. Try 30 and see how it feels. If it feels like the tires aren't gripping and bouncing around, let some out, and if they feel squirrly, add some... but only in 1 or 2 psi increments. A little goes a long ways with DH pressures. And be warned, you can pinch-flat a UST tire riding DH... I've seen it happen.