That tree is substantially larger than the one I’m building - mine is a small tree. I believe this is more like the size of Ebihara’s zelcova that I posted about in that thread. He has some photos about where he chopped that tree. Study those to get an idea of where to chop yours. Just remember that about ~2/3 of the tree will be above the chop to get the proportions right. So if you chop a foot and a half above the ground, you’re building a 4-5’ tree. So don’t be shy when you chop and keep your final vision in mind.
Couple of other things. As Sorce said, find the nebari first. You might change your mind about where to chop if it ends up being 6” below the soil surface. Second, look for the straightest trunk you can find with no bends or curves. A tree this size will be a substantial commitment of time, so make sure you start with perfect material so your not disappointed a few years down the line.
Looks like a fun project - get at it!
S