00'. That equates the allowance for compressibility. This is important because aircraft stall relative to equivalent airspeed and not CAS.
Remember also that high speed buffet is in relation to Mach number but low speed buffet although it may be expressed as a Mach number actually happens at a EAS.
There is more to it than that. At high angle of attack (i.e. close to buffet/stall) Mach/compressibility effects start as low as M=0.3.
Therefore the low-speed buffet boundary at altitude is not constant EAS.