The Ashby crowd will hate this, but here it is: 15% FOC is plenty for whitetails. Going to 19% costs you arrow speed, sight tape, and forgiveness past 40 yards in exchange for penetration you don't need on an animal that doesn't have the bone mass to justify it.
I'm not saying high FOC is wrong. I'm saying it's a tool, and like every tool, you have to know what you're using it for.
The Ashby work is right, but not for what people use it for
Dr. Ed Ashby's penetration studies were done on cape buffalo. Buffalo have shoulder bones the size of a kid's forearm. Getting through that bone with an arrow requires concentrating mass at the tip, sharp edges, single-bevel rotation, a thin shaft. Ashby's work is the gold standard for what it studied.
It just doesn't apply 1:1 to deer.
A whitetail's shoulder isn't a buffalo shoulder. The thickest bone on a broadside deer is maybe the size of your thumb. Modern broadheads from any tier-one brand will punch through that with a 12% FOC arrow at 65lb just fine. Adding another 5% FOC isn't unlocking some hidden penetration. It's solving a problem you don't have.
What you give up to get high FOC
Speed. Heavy arrow flies slower. Your trajectory drops faster. Past 40 yards you're holding noticeably more pin, which matters when you mis-range a buck at 47 thinking he's at 42.
Forgiveness. Heavy arrows are more sensitive to release errors. A clean shot with a 480gr arrow and a clean shot with a 380gr arrow look the same. A torquey shot with a 480 wanders further left or right than the same torquey shot with a 380. The heavy arrow is slower to recover.
Sight tape range. A 13-grain-per-inch FMJ + heavy insert + 175gr head gets you a sight tape that runs out of pin at 70. Same bow with a 9 gpi shaft and a 100gr head, you've got pin to 100. Not a problem if you only shoot 50-yard whitetails. Big problem if you ever want to stretch out for elk.
Where high FOC is actually right
Cape buffalo. Yes, the original.
Elk, especially big bulls quartering. Heavy bone, hide thicker than your finger, longer shot distances common, you want all the penetration you can get.
Moose. Same thing, more.
Hogs, especially if you're targeting big boars with the shield. Anything where a deer-class setup might glance off something it shouldn't.
Trad bows under 50lb. Need every advantage the arrow can offer. High FOC makes a slow arrow penetrate more like a fast one.
Where it's wrong for most hunters
You're a Midwest whitetail hunter shooting 60lb. Most shots are inside 30 yards. The biggest deer you'll ever shoot weighs 200. A 410gr arrow at 290fps with a 12% FOC and a 1.5" cut mechanical kills that deer dead, every time, with a margin of speed and forgiveness you'd lose chasing higher FOC.
You're shooting indoor 3D. Stop. Run 11% FOC and chase points. FOC is irrelevant inside 20 yards.
You're an antelope or Coues deer hunter. You need speed and flat trajectory more than mass. Drop the front weight, run faster.
The honest middle
For Midwest whitetails, 12-15% FOC is the sweet spot. Enough mass forward to stabilize broadheads. Enough overall speed to keep your sight tape sane. Enough forgiveness that a slightly torquey shot still hits where it's supposed to.
If you want to play with FOC, do it on purpose. Match it to the animal and the shot distance. Don't chase a number because the Ashby crowd shamed you into it.
The Arrow Recommender in The Forge lets you pick a priority: max speed, max penetration, balanced. It builds the right arrow for your animal, not the right arrow for an internet forum.