Many guides will tell what altitude can find stuff in. I was a little confused of what altitude is though, is it the vertical distance from the mantle? So that the first block over the mantle would be altitude 0?
hit F1, then move your mouse to the top left of the screen and hit “menu”. From that menu, open up the debug info. It has camera position and player position including coordinates and altitude. If hitting F1 does nothing, you may need to enable the menu within settings > game. And yes, it starts from mantle and goes up.
alternatively press ‘P’ to go directly to current location in player’s menu - it shows coordinates and altitude
The mantle is not smooth. Mantle blocks range from 0-4 in depth. There is an actual zero altitude that’s consistent no matter how deep the mantle is in that spot.
The easiest way is to just open your places menu as suggested here.