Ok, I always wonder this, so please help me cement it in my brain. This recipe calls for 6 pounds of corned beef brisket which is cooked for 4-4...

Tbrooks, I used a 3-lb. corned beef a couple of the times I tested it, and the recipe halves very well. The simmering time depends mostly on the thickness of the piece of corned beef you're working with, as you mentioned, which can be similar whether you're working with 3 or 6 pounds of brisket.

So if it's a skinny-looking point cut piece that tapers into a thin triangle (about 1-1.5 inches thick, like the one in the recipe slideshow) it very well might need only 3 hours of simmer time, so just check it for tenderness early (and it should hold well between the simmer and bake stages if it does finish early).

I'm not sure that I am understanding the question, but for a cut of meat that is half the size I would start at half the cooking time, and then, very unscientifically, I would check the meat periodically to see if it's done. Alternatively ( and I am a big fan of this method!) I like to talk to my butcher about what I plan to do and get his/her insight about any alterations I might be making to a recipe. I hope that is helpful!

I have a 3 lb in the oven right now and I plan on cooking the same amount of time but I do mine at 300, I get around the same size every year and find that it takes at least 4 hours if not a bit more. IMHO!