Sounds like you got good advice. I always hand ream the last few thousandths of my chambers. I would snug the extension hand tight like you did in the video, turn the reamer a few rounds, clean the chips from the chamber and try the go gauge again. Repeat until you get the bolt to close on the gauge. Then torque your extension down to spec and check the headspace once again. You may have to ake just a little more out once the extension is torqued. Also remember, in a semi auto you want a more generous chamber since you will have no mechanical advantage when chambering a round. (No, the forward assist won't help.)
One other point, I always chamber for the 5.56 Nato or Wylde chambers where the customer/shooter will use surplus ammo. There's lots of controversy surrounding the use of 5.56 Nato ammo in .223 chambers. I choose to err on the side of caution. The Nato and Wylde chambers are safe for both types of ammo.
