It's definitely a bit arbitrary. You will get all sorts of opinions and methods.
Ignoring that, there's a harvest window that isn't very specific, either ... it will come down to personal preference. You may need to test different points in the harvest window to figure out exactly what you prefer. You can do this with entire harvests over time, or takes some buds early, middle, late off of one harvest - be sure to label them for your smoke test later. You can still take the bulk when you think it'll be best, but take a couple buds early, maybe let a few buds mature longer etc...
Trichome coloration is probably the strongest correlating factor you can use, but it's wise to use more than 1 observance to be consistent.
trichome coloration - ignore trichomes on lneaves. Trichomes on calyxes only.
pistil coloration / retraction
bud density -- this one should be the last thing you check and as limited as possible,but a good way to verify nonetheless
10% amber? 50%? this is where personal preference comes in. Heck, some people like it approaching 100%... but that's less common.
Some plants won't amber up. It's some sort of genetic trait or whatever, but it happens. So, when you encounter a plant like that, waiting for amber is only going to lead to over-ripened buds.
I used to 100% rely on trichomes, but i've changed that strategy the last few harvests and I like what i am doing now... you may not, but this is what i do... Once the plant is fully fattened up, i start scoping trichomes. I wait for ~10% amber on the top colas -- this would be considered an early harvest. If that doesn't occur within 7-10 days of the buds fully fattening up, i harvest no matter what. I'm pretty patient about what 'fully fatten' means.. it's beyond a doubt not increasing in girth day-to-day, so i may err on the late side in that regard. So, there's a little subjectivity involved, but an individual can be consistent about it. relative to your perceptions/judgment you may find ~14 days is a better maximum.
Anyway, this avoids the rare plant that doesn't amber up from over-ripening. I really don't like the taste of buds that are extra ripe. You may feel differently about that, which would make my process useless to you.
be consistent in methods. adjust as needed. within a few harvests you'll work out something that keeps you on target and consistently harvest at a particular point that fits your preferences..