I don't believe that there are very many hunters that can get closer than 8"-10" with just a single encounter. Even bucks that I know can be a challenge to judge. Some days/angle/lighting they look entirely different than the last time I saw him. Multiple sightings and multiple pics help towards being accurate.
I've been hit and miss with judging on the hoof. I once had a neighborhood buck that I estimated at 140". I eventually got to score it and I came up with 140 1/8". Not bad. But I've also been off by 10" on a few others.
A 10 point has 19 measurements. Being off by only a half inch on each one can really effect your estimate.
Length measurements are the biggest factor, mass is 2nd and spread is 3rd.
Beam length can be a challenge. Spread and mass are easier. For mass, I like to compare the base to the eyeball. Spread is fairly easy...a mature buck's tip to tip ears spread is about 16 or 17 inches. Tine length can be compared to ear length.
I like to put them in classes...shooter or not a shooter, then score it when I have the rack in my hands.
I have never given a dried fig for measurements. That changed on the SOA hunt last year, where they told me in no uncertain terms that if I messed up and shot a baby I was banned from the drawings for life and would be kicked out of the hunt.
Minimum requirements were an 18" inside spread or a 20" main beam, I believe. Maybe vice-versa? Basically a 4yo deer or better. The biologist stressed that I was looking for either a main beam 3 times the length of the ear, or antlers spread wide enough to put his alert ears INSIDE the rack.
The buck I shot barely made each criteria, and was kinda shrugged off by the state guys.
The one my dad got was impressive enough by their standards to go on the state website.
All that to say, I definitely believe in using ears as measuring tools. I will shoot every buck I see that goes outside the ears and/or has a main beam 3x the length of his ear. The first measurement is much easier to ascertain if you have the right angle, in my opinion, but the second is probably more reliable overall. Many of the deer from the area we hunted were high and tight racks.
No idea what our deer scored, but they were both aged by the biologist at 4.5 years. I call it a win. I'm not smart enough to do the math on inches, but general rule-of-thunb guidelines like the biologist gave us will be something I bear in mind in the future. Although honestly, i made the call to shoot based off the deer's body shape and the way he carried himself. He was way more beef steer than goat, ya know?