Measure against the screen edge
Place a card, paper, or small part flush with the top or left bezel so it lines up with edge ticks. Recalibrate if ticks look wrong.
Guides
How to get useful measurements
Practical notes for the calibrated ruler on phone: physical accuracy first. Open ruler.
Selection & paired guides
Select mode reads width, height, diagonal, and area. Guides are better for repeated spacing: drop two lines and read the distance as you drag.
Keep zoom at 100%
Browser zoom multiplies CSS pixels. After calibration at 100%, pinch-zoom or Ctrl+/- will skew results.
Craft and small parts
Prefer millimeters for beads and findings; switch secondary readout to inches when sharing with US collaborators.