The 'short enough to trust' constraint is the real design rule: a default board with forty tiles trains people to ignore all forty. Deploy deltas sitting next to the vitals is what turns a chart from decoration into a cause.
A default board with forty charts is how people learn to ignore every chart. What turns a number into a cause is seeing how LCP, INP, or CLS moved after a release, on the same timeline as that release, and only opening a ticket when the move holds across more than one scheduled run. One noisy lab run beside a still-green budget is still decoration.