Find a speedbump (speedhump) and roll your board over it (without standing on it)
If it comes within 5mm of it, it’s too low. Any higher than 15mm, is too high. 
So what I’m saying is that it totally depends on wheelbase, longer wheelbase needs more clearance. But also, it depends on WHERE between the trucks you’re referring to. You need an arc of clearance. If it’s right next to the front or rear trucks, it can be closer to the ground. In the middle is where you need more clearance. And that’s assuming no flex. The arc needs to be exaggerated if the deck flexes.
When you roll over obstacles, curbs, and rough terrain, the further you are from the truck, the more clearance you need. So the longer the wheelbase, the more overall clearance you need if the bottom is flat.