Questions about head work.

I believe your auther is referring to the angle and throat directly under the seat angle that would normally be part of a 3 angle valve job. You do NOT want to enlarge the throat to the diameter of the seat angle cut. There needs to be an approach to that angle, normally created on a 5-angle valve job by the throat and top cuts. Those help the mixture turn without going turbulent and makes a big difference in low and mid-lift flow. Blending right to the seat angle sacrifices low and mid lift for high lift only, which is not a good sacrifice in most engines. For what you have here, I would not do any major enlargement other than to smooth the transition from the port roof around the guide boss and the throat. Leave about 1/4" of that taper above the seat angle untouched and remeber the trick is smooth transitions, not a step or low points as it changes shape. Less is more here. The last thing you want is a lot larger volume of space at the bowl and throat just ahead of the valve head.