The starting corner is means "Where is pixel #1 on this model?"
The location of strand #1, is up to you when you physically build the tree.
If you want strand #1 to be at 3-o-clock, then you'd build the NC model with starting corner as either top right or bottom right depending on where pixel #1 is on your string, and you'd set it up as a 180 tree.
A 270 tree visually puts the first and last strands at 10 and 2.
A 360 tree visually puts the first and last strands at 12:00
The 90 is at roughly 8 and 4
Hope that helps.