I am agin the idea of default
I’m a little worried by this.
The cg position determines the amount of “trim”, the amount of “trim” has an effect on drag and in conjunction with Cost Index I would expect to see some influence on optimum cruise level. But the proper reading of cg position is to understand how it relates to longitudinal stability in the first instance. Stability first, cruise second. And if an FMS wakes up with a figure of 4% when you first hit Perf Init, don’t believe it. You won’t beat the Wright brothers first attempt at flight, you won’t reach 35ft and the resulting fuel burn will be anything but economic. I'd take an EWAG that the majority of aircraft are at their happiest 25%mac plus or minus 10%. And I always liked to enter the figure as determined by the proper method. I rather liked the 737-400 by the way and the 767-300 too but they didn't have retro winglets in my day.