But the freeze/boil numbers are arbitrary as well only being true at sea-level. Where I live in Colorado, we're 1 mile (cough, 1.6 km) above sea level meaning those formulas are meaningless.

They aren't meaningless. Water boils at 95 C at your altitude - effectively the same temperature for all "common" usages. E.g. you'd still better not stick your hand into boiling water.

A <5% error for the zone that >99% of humans inhabit seems pretty practical to me. The freezing point is effectively identical.

iirc, water boils at 95C is around 5000m altitude, no one lives normally there - even La Paz is less than 4000.