Mentioning a number of millimeters IMHO doesn’t help, because it highly depends on wheelbase as well. Shorter wheelbases can have much less clearance and still be just as functional. A long wheelbase needs a taller clearance, or a clearance that’s arched in the center.
My goto test is I go find a speed hump local to me, one of the nasty ones, and I like the skate to just barely be able to roll over it head-on with me on top, without scraping (but a couple millimeters away). The shorter the board, the lower it can be.
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.