In theory if you knew all variables you could calculate the forces required to flip you over the bars, but there are so many variables: the force of braking, the coefficient of friction on the road (which prevents the front wheel from skidding) the location of your centre of gravity (which will move forward as you brake), the angle between your centre of gravity and front tire contact patch, the angle of your arms to the handlebars, the wheel length of the bike since a long bike is less likely to endo, any load you may be carrying on the rear rack, whether your feet are clipped in or not, etc., etc.