The "how far above grade" question is not too tough. Every tree in nature starts out with a seedling or a sucker coming up at the ground level. The root flare is exactly at the level of the soil.
After that, the roots and the root flair gain diameter, and the tree apparently rises out of the ground, curiously bringing the soil level up with it. As you might guess, the roots are displacing soil, and the uphill rise begins, mostly due to atmospheric carbon being pumped into the ground to make roots with.
Plant seedlings at the ground level. Small trees...about the same, depending on soil type. If you are planting a really big tree, then you need to guess how much the tree would have raised itself out of the topsoil at the nursery it came from, and then plant it that way.
Myself, I just always take the existing root flare, and plant it at soil level.