Or should it be (35 cal / 30.3 cal)^2 = 1.33 ? Nominal bore diameter rather than groove diameter.
I agree that cross sectional area is good to consider. It's cumbersome to work with though, especially when applied to modern expanding bullets.
My results over the years seem to indicate that with expanding bullets, bullet performance is a far greater variable than bore diameter.
Don't get me wrong, all things equal, a bigger bullet makes a bigger hole. But in the field, no two shots are the same. Most of the wound channels I have seen are conical in nature. Small entrance, lots of bloodshot on the far shoulder whether I get an exit or not. Often this is not apparent until butchering the animal. Might have the bullet sitting just under the hide on the far side, in what looks like a toonie sized hole, but have to cut a 6" circle around the wound to get all the bloodshot out.
Even shooting solids that do not expand - usually they exit, but pick up bone and tissue on the way and make a conical wound channel.
With frangible bullets, same thing, just no exit and WAY bigger cone. I have witnessed exception to this rarely with round nose bullets. This is something I seldom see mentioned, but I have seen entrance wounds that look like exit wounds simply due to the hydraulic response to a bullet expanding.
A small hole directly over the heart will have much faster effect than a big hole through both lungs.
With this in mind, I think most of us can agree that success is contingent on shot placement and bullet performance. Little more.