I used to do mental calculations in my mind when I would look at a deer, even through a spotting scope or on standing out in the field in front of my stand or whatever. I would estimate tine length and beam length, then double it and add the spread and 20% for mass for a pretty close guess. If it was a real heavy rack I might add a couple inches and if it was a skinny rack subtract a couple inches after adding the 20% for mass.
After a few years of doing this, I mostly started just guessing them as I saw them and then adding them up. Turns out my first impression guess was usually pretty close to my addition.
The buck in the pic has average mass, average tine length and a small spread (probably 15 inches) but has long main beams. Without doing any addition, I would say he is around 150 gross and pretty darn symetrical so he'll net between 145 and 150. As far as age goes, IMO he is probably 3.5.