I have been field judging deer for over 15 years. I've gotten much better at from tons of practice. When I'm trying to judge one in a hurry before he takes off, I use the following methods.

1) Score one side, # of points and their length.
2) Estimate mass, 14, 15, 16.....18 etc.
3) Beam length, 19, 20 ......25 etc.
4) Add 1-3 and double it.
5) Add in estimated inside spread.

For me, mass estimates come from being familiar with known deer that I've measured. 15 - on the slim side, 18 - on the heavy side. This comes from experience with the deer from my area.

Beam length, again, is from deer that I've measured. I have a picture in my head of what a 22 or 24 looks like. This one can be difficult because there many variables. Ex. Spread, how much curl and how far towards the end of the nose the tips are when veiwed from the side.

Inside spread, this one is a bit easier. In my area, 2 1/2s ears are about 16" tip to tip. 3 1/2s are about 17 and 4 1/2 s and older are about 18"s t to t.

From practice, I take a mental picture of the deer and can mull it over in my mind after he's gone and come up with a pretty accurate score.

If things are happening in a hurry and I have to decide to shoot or not, I've got pics in my mind of what 120, 130, 140... 180 class deer look like.

It's always the best if you take one look and go "holy S---T!" and just get ready to shoot!