Great advice..... And yes its a pretty common measurement .. And its a great starting point to field judgeing a deers rack..

Start with the inside spread (even with ears around 16 inches) if its outside of ears its easy to get a close guess how wide he is by starting with that 16 inch and adding to it for total inside width. Then look at each tine, and guess the lenght, by using that inside spead as a gage

Say you see a deer even at his ears.... 16 inch spread
Deer has brow tines that look like a 1/4 of that spread, 4 inchsx2
Deer had g-2 tines that look like they are half of the spread... 8 inches x2
Deer has g-3 tines that are one third of the spread....say a bit over 5 inchesx2
Say deer looks like each main bean is the same lenght as the spread is wide....16 inches x2

Then here comes the guess, you have 4 mass measurements on each side one at the smallest area around the base, one between the brow tine and g-2, one between the g-2-g3, and one more measument between the g-3 and the main beam point..
say you came up with 30 inches of mass, now add all that up in your mind...

16"
4" x2
8" x2
5" x2
16" x2
30 " of mass

And now you have a deer's gross score of 112 inches, it takes a while to get good at it , just look at a lot of racks!!

Once you see alot of 100 inch racks you can forget about alot of the adding, it will come natural to you...