Hardware
Measure, don't eyeball
Geometry, timing, thresholds: every constant traces to a spec, a calibration, or a measurement you just took.
Physical quantities are measured, never judged by eye. A dimension comes from the spec, an axis from the model file, a threshold from the calibration you just ran; your visual impression is not an instrument, and neither is the machine's. The general law says every constant needs a source you can paste; the bench version adds that the source is frequently something you must go measure, which is not an excuse to estimate instead.
Diagnosing by appearance is the same sin at higher stakes. The part looks misaligned, so nudge the offset: that is tuning a constant to hide a bug you never located. Find the wrong value in the chain, fix it at the source, and let the geometry be right because the numbers are right. Renders and photos are for catching that something is off, never for deciding by how much.
How to practice it
No instrument, no number.
If the constant didn't come from a spec, a config, or a measurement you can name, it isn't a constant. It's a wish.
Locate before you compensate.
A nudged offset that makes it look right buries the wrong value that made it look wrong. Trace the chain.
Renders detect, sources decide.
Use your eyes to notice the gap; use the model file to learn its size. Never reverse the two.
Re-measure after change.
Any edit upstream of a calibrated value expires the calibration. Cheap to redo, expensive to trust stale.