I don't agree that you can skip steps 3-5 before doing step 6 — it's true that the vertical error should be minimal if the edge+end angles are close to 90°, however, there is still an error.
εz = sin(εθ) · z In plain English — the vertical error is equal to the sin() of the angle error (the amount it differs from 90°) times the height. For example, if it's 5° off, the vertical error should be 8.7% of the height, which isn't terrible, but it is still something.
