Do you measure from the "lug slots" where the bolt sits, or do you measure from the end of the chamber??

Barrel length is measured from the breech to the muzzle. Lock a bolt into the breech end, make sure there is no removable muzzle device attatched, insert a dowel or rod into bore until it touches the bolt face. Mark a line on the rod at the muzzle, remove & measure.

Barrel length is measured muzzle to breech face on any rifle or handgun.
For simplicity sake, on an AR-15 rifle, you can measure the complete barrel and subtract 3/8" as that's the nominal dimension from the breech face (bolt locked in battery in the barrel extension) to the rear of the barrel extension.

If you are shortening a barrel, just keep an extra 1/8" or so on there so there's no question about legality.
Most 16" barrels come from the manufacturer's at 16.25" length if you actually measure them...

Randall Rausch
www.ar15barrels.com