From where to where is the actual length of the barrel?  Say, I wanted to cut my 16" barrel down to 14.5" and then thread it, where do I measure from?  Do you include the entire piece (ie the feed ramp) or just from the begining of the chamber?

The easy answer is that barrel length is measured from the breech face while unloaded and locked closed.

What you are really asking is "where is the breech face so I can measure from it?"
Some important information to give you context:

The barrel extension threads on to the barrel.
The bolt locks into the extension and the bolt face is the breech face.
If you take a barrel extension and bolt, put the bolt in place so that the locking lugs bear on the extension lugs, then measure the depth with a depth micrometer, you  get 0.750" plus or minus tolerances.
So, the breech face is 0.750" from the torque shoulder on the barrel.
Now it gets easier.
The barrel extension is 1.125" long, or 3/8" from the end of the extension to the breech face.
So, you can measure the overall length of your barrel and subtract 3/8" and you will have the 'legal definition' length.

So, to cut your barrel back to 14.5", you will want to cut it to 14 7/8"

I also do barrel work if you want this done...

Randall Rausch
ww.ar15barrels.com