Short answer .... its a matter of taste according to grower .... Some strains don’t amber very much , some will shoot for a 80% milky trich average.
Amber is just telling you that heads have peaked and are converting to CBN - relaxing mood , couchy high.
When i run autos or any plant actually, i look for overall percentage of milky triches AND pistil recede ( which shows calyx maturity) .... hell many strains still throw white pistils at end ..... its just the percentage OVERALL i use to determine- even if i have to partial harvest the mains and let the lowers harden up.
IMO waiting for amber is really not a reliable benchmark. There are other signs plant is ready .