?

My rough measurement shows ~ 5 1/2" of barrel forward of the FSB.  You need to leave ~ 3/4" for muzzle threading, so say you take ~ 4 3/4" off the barrel.  Your overall length will be around ~15 1/4".  The Vortex adds ~ 1 1/2" or so of length to the barrel when permanently attached, so you would be good on length with an overall length of ~ 16 3/4" (give or take based on my rough measurements).

Basically you can take it right to the FSB and be good with length.  Experience has shown that the gas port will have to be opened up a little to allow for proper gas system operation when trimming a 20" barrel like this.