Where does the rule come from?

Distance ÷ 10 for a 25-module code, scaled by module count.

Does the quiet zone count?

Add four modules each side on top of the minimum.

Measure from the pavement, not the driver's seat.

Set the distance people will scan from and the module count. The minimum printed size comes out in millimetres.

Scan distance50 cm

50 cm · 25 modules

50 mm

Modules

minimum size50 mm

module width2 mm

with quiet zone66 mm

At 50 cm a 25-module code needs at least 50 mm across. Print the proof at that size and scan it from 50 cm in the light it will actually hang in.

Where does the rule come from?: Distance ÷ 10 for a 25-module code, scaled by module count.
Does the quiet zone count?: Add four modules each side on top of the minimum.
What about vehicle wraps?: Measure from the pavement, not the driver's seat.