Yep, deck angle, which directly relates to AoA, in the cruise.
Now, for an example, let's look at the Lockheed TriStar, the standard body model, not the -500.
During flight test, it was found that at the 'best' altitude for the most efficient cruise flight, the deck angle was, 3.2 to 3.5 degrees.
In addition, for the respective optimum deck angle, the desired cruise speed was on the order of M.845.
Slower, resulted in more fuel burn, for the sector.
Faster, also higher fuel burn
but not as much as flying slower.
The TriStar, you see, was designed as a high speed cruiser, from the outset. It was also the first wide-body civil jet transport with a true laminar flow wing.
The FMS fitted to the -200 TriStar (a well designed Litton unit, installed by SaudiArabian, on all their TriStar aircraft), offered LNAV, VNAV (for climb/descent, but excluding planned crossing altitudes) and thrust management, and included software to enable the pilots to determine the optimum cruise altitude, dependant on winds, temperature aloft, weight, cruise speed desired, fuel price, etc and also offered a step-climb function, which was generally disabled, when the aircraft was close(er) to the planned destination.
How close?
Usually within three hundred miles, or so, IE: the fuel used to climb 4000 feet (500KG, approximately) was more than staying at the present altitude, to the planned destination.
The formula used by the software?
I don't know, but the FMS worked to perfection.
Lockheed/Litton....way ahead of everyone else.
Having said this, on the B707, specific flight planning charts were provided, which provided the same general information in tabulated form.
These were called 300fan flight conduct charts, and worked well.