Our friend BOAC above described best procedure and to apply for Vra/Mra.
Vra/Mra = turbulence speed or Mach number.
Yes, the 707 and 727 were at 280 KIAS/Mach .80 as well.
The 747-200/300 is 295/.835... These are "book figures".
A better speed/technique to be flown is as follows -
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Obtain high speed and low speed buffet margins figures.
Place two "airspeed bugs" on such speeds or Mach number in cruise.
The higher the level, the narrower the spacing between the bugs.
Depends also what buffet margins you use - i.e. 1.3G or 1.4G.
In severe turbulence, target Vra/Mra is mid-point between bugs (bisector).
Example - 302/267 hi/low buffets = Vra 284.5 KIAS.
At lower levels, low speed buffet is replaced by "stall speed - flaps UP".
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Happy contrails