The Buffet Margin
An altitude can be reached where there is only one speed at which the aircraft can fly. In the case of a 1 g s, this altitude is called the ‘Aerodynamic Ceiling’.
Operating an aircraft at its aerodynamic ceiling would leave no safety margin. In 1 g flight the aircraft would be constantly on the point of stall.
Regulations require an aircraft to be operated with a minimum buffet margin of 0.3 g.

Buffet Onset
To more accurately calculate the high and low speed buffets or the buffet boundary, a buffet onset chart found within the aircraft flight manual is used.
The chart provides the values of the Mach number at which low speed and Mach buffet occur at various masses and altitudes.