For gravel on a mountain bike with tubes, I would stick around 30. For hard pack I would go up to 35. With tubes I had to run a higher pressure. But you don't want to pinch flat on tubes, so bottom line 30 or so. You need a firm tire but a little bit of squish in them for traction. Hope that makes sense, it will once you ride a little more. And since your LBS may know the exact trail ask them what they think as well.