Where the rule comes from
The 1 mm per metre rule approximates the resolution of the human eye at around one arcminute. At 5 m, a P5 screen has pixels roughly one arcminute apart, so the grid disappears and the image looks continuous. For video content, that is a genuinely useful threshold.
Failure one: text
Text needs three to four times the resolution of video to look clean, because the eye is reading edges rather than interpreting motion. A P5 wall at 5 m is fine for a brand film and unacceptable for a spreadsheet. If the screen will show data, financials, CAD or menus, halve the pitch before you halve anything else.
Failure two: ambient light
In a bright room, contrast drops and the eye stops resolving fine detail long before the pitch limit. A P1.5 SMD wall in a west-facing boardroom at 4 pm looks coarser than a P1.8 COB wall in the same room, because the COB panel holds black and the SMD panel goes grey. In daylight rooms, packaging beats pitch.
Failure three: shaped screens
On a curved or spherical surface the effective pixel density changes across the face. Near the silhouette edge of a sphere, pixels compress in visual angle, so a P2.5 sphere that looks smooth head-on can show banding at the rim. For spheres we usually step one pitch finer than the flat-screen calculation suggests.
A better starting table
Video, dim room: 1 mm per metre. Video, daylight room: 0.8 mm per metre with COB. Text and data: 0.5 mm per metre. Spheres and tight curves: one step finer than the flat equivalent. These are starting points, not specifications — the content and the room always get the last word.