You are correct.

The closer the backspacing is to 1/2 of the rim width, the closer to 0 offset you will be. So, an 8" wide wheel, with a backspace of 4" will stick out further than the same wheel with a backspace of 4.75"

Lay a wheel face down on the ground, and then lay a straightedge across the edges of the rim (parallel to the ground). Backspacing is found by measuring the distance (vertical now) from the mounting pad to the bottom edge of the straightedge.

Brake caliper clearance is determined by spoke design and wheel diameter -backspacing/offset mean nothing here.