You usually only discuss letter spacing when it comes to Headings
Headings with crispness use negative letter-spacing. (usually -0.5%, -1%, -2%)
No correct way?
There’s no correct way to achieve perfect spacing atm.
In CSS
In CSS, the best unit for letter-spacing would be em.
Below it’s what Tailwind use for their system:
--tracking-tighter: -0.05em;
--tracking-tight: -0.025em;
--tracking-normal: 0em;
--tracking-wide: 0.025em;
--tracking-wider: 0.05em;
--tracking-widest: 0.1em;