MTB seat height
 

I'm a road biker and I like my saddle very high, at the max.

How do you determine the height of the seat for a MTB? Sitting on the saddle, should my feet be able to touch the ground?

Level seat or slightly tilted up?

What does "at the max" mean? You know, they do make seat post of different lengths. How high is your seat on your road bike? What type of leg extension are you getting (besides "at the max")?

I'm new to the mtn bike game. When I'm on easy trails, I set my seat just like the way I do on my road bike.

When I get on rougher roads, I drop my saddle 2 - 3 inches.

When I'm horsing around (practicing bunny hops or wheelies), I drop my saddle 5"+

You can't adjust your saddle with the same concept as a road bike. Each time you raise & drop your saddle, your KOPS position will change as well. On a road bike, once you find your sweet spot, you almost never have to touch your saddle again.

On a mountain bike, I change my seat height a few times on the trail depending on terrain.

As a roadie myself, I understand what you're thinking. But you're thinking mostly wrong. The reason your MTB has a quick release seat post and your roadie probably doesn't is for a reason. The saddle height depends on how you ride, and on what terrain. If you're doing a lot of XC with little gnar, then having the seat high (like on your roadie) means you have the best pedaling efficiency which is good. Your knees won't tire out, and your back will be more comfie. If you're doing jumps, going down hills or over bumps, you're going to want your seat lower so that you can stand up and get your ass over your rear wheel to maintain balance. That's why XC bikes are pictured with the saddle high, and dirt jump and downhill bikes have the saddles slammed all the way down.

A good feature on MTB seatposts is numbered gradients.
That way you can move the saddle between numbers for road height and mobility height.
Useful for when you get to the end of the flowy singletrack and enter the rock garden. And vice-versa.