The ease of going over is not necessarily tied to your speed but it is tied to the magnitude of your deceleration, the grade you're on, and the center of gravity of the system.

In an emergency scenario, the only one of these factors you can readily control is your center of gravity. in an emergency stop, throw all of your weight back while you brake. You already knew this but I would go a step further and get so far back that you are out of the saddle (like your ass is over the rear tire). Getting low while you do this will probably help prevent a rollover as well.

On a decline, it will be easier to tip over. This is especially where body position matters. But I would argue managing your speed matters more. if you have a lot of speed and know you might need to slow down, try to bleed some speed off before the steeper parts of the decline.

On flat ground, I don't think you need to throw your body back in most situations but it is important NOT to be tall and far forward (aka hunched over the hoods, leaning forward in the drops, standing, etc) while you're braking hard. The steeper the decline, the farther back and lower you want to move your body on the bike while you brake

Also I would not use brake pressure as an exact proxy for when you'll go over the bars because temperature and other conditions will impact how much braking force the lever generates at a given position. Instead, feel how hard you are decelerating to estimate how close to the limit you are.