Welding Tips and Tricks

Using flux-core---does it mean wire is flush with the contact tip hole and the measurement is from the tip to the work or is it measured from some amount of wire stick-out to the work? I've read about it with gas shield, but I'm unclear how the distance/standoff is measured with FC.

The CTWD is literally the end of the contact tip to the material. There's an explanation of CTWD and its affect on the weld here:
http://www.esabna.com/euweb/mig_handbook/592mig7_3.htm