Should have qualified that as MY OPINION - that ear width isn't the most reliable way to judge width, though it may be the quickest and if you have practiced enough, you can probably be right most of the time, though I would wage that most people would be way off on their width guesses if they only use ear width as a standard. And the reasons why are obvious to me.
First deer don't always have their ears perfectly straight - angles are deceptive and deer can put their ears in a lot of different angles, thus leading you to believe that they are wider or more narrow than they really are.
Second, ear width can vary greatly as seen by the different measurements reported here. A lot of different factors can determine width too - antler angle, pedicle separation, skull measurements etc.
Third, the good old "out past his ears = 30 inches" standard has been time and again been proven to be wrong 99.9% of the time. Too many folks rely on this and I am sure that is why they see so many 30 inchers yet very few true 30 inchers are ever harvested.
So what is the best way? Well, rope the deer, wrestle him down, pull out your measuring tape and measure! If he is big enough, let him go, shoot him and then you won't be disappointed.
Truthfully, the best way is to look at a lot of deer with known measurements (taxidermy) and then compare the widths - practice and train your eye the same way you train your eye to approximate distances. After you have seen enough of the same thing, you can pretty well judge something that is like it.
You can use surrounding brush or trees to help you estimate. Calculate how it compares to the objects around it. Use contrast too
And laslty - you can still use the ear width, I wouldn't dismiss it altogether as stated previously it does have some merit and with practice, you can be pretty accurate with it. After all, it is the closest thing to the deer's antlers. But I wouldn't go over 22 inches. Yes this is a conservative guess on my part, but I would rather be wrong and short on my guess than long and disappointed.
Also, to me width is really overrated. (Sorry Hunt_Rez!) There are a lot of other factors to look for when field judging.
UTROY
Proverbs 21:19 (why I hunt!)