Hi
GFCH, welcome to PPRuNe!
I speak for the B777, which I believe to have the same FMC VNAV philosophy as the B757/B767. I'm sure that qualified B757/B767 will correct me if I am in error...
OPT : This is the Still Air (or zero wind) Optimum Altitude for your present Weight, and the Cost Index which you've entered. It is not permitted to exceed
MAX. Useful if you have block levels and there's no significant Vertical Wind Gradient.
MAX : This is the most limiting of your Thrust Limited Altitude (Temperature), Maximum Permitted Altitude (AFM Limitations), or Buffet Protection Altitude (Usually 1.3 G as you've quoted, but some jurisdictions may allow it to be as low as 1.2 G).
RCMD : Recommended best level for your Cost Index entered, Step size entered, and Wind Component. With respect to the Wind component it
IS very important to insert a reasonable range of winds above and below the Optimum Altitude such that the FMC may compute the best Wind/Altitude trade available.
Recommended is usually the level to fly at, but be aware that
the FMC has no knowledge of actual or expected turbulence, and may push you all of the way up to MAX. MAX will only protect you for light turbulence, whereas you will need protection up to 1.5 G to 1.6 G in Moderate Turbulence conditions. For the B777 (non -300ER) this is approximately 1700 ft below MAX, get to know your own type characteristics of how much margin is desirable below MAX for anything more than Light Turbulence.
Hope that that helps, standing by for responses from real B757/B767 operators!
Regards,
Old Smokey