Welcome aboard, Teddy — it's a good question, and not a strange one at all; anyone thinking hard about a frontal shot ends up asking exactly this.
The honest answer is that there isn't one "thickness" to give you, because it isn't a single slab of bone. On a mature bull the frontal is a sandwich: an outer plate, then a honeycomb of sinus (air cells), then an inner plate — and only behind all that does the brain sit, smaller and lower than most people picture, roughly on a line level with the eyes rather than up between the horns. Above the eyes, where you're pointing, you're mostly into that sinus honeycomb rather than solid bone, so it's less about a measurement in inches and more about how much structure a bullet has to stay straight through before it reaches anything vital.
Two things that matter more than the number: on an old bull the boss adds inches of solid horn over the top of all that, which is why a high frontal glances or stops short; and it's why a frontal brain shot wants a heavy, well-constructed solid on a very precise line — centred, level with the eyes. A soft, or a shot placed high, is trusting that honeycomb to do you a favour, and it won't.
If you can get your hands on an old euro-mount and look at it side-on, it tells the whole story better than any figure I could give you.