For some reference, 1e and 2e use a 30 second round broken into 6 phases.
1e figures CUF as function of combat experience which itself is a function of rolled attributes; the more your attributes total, the younger and less experienced you are. Low CUF is good as CUF is the number of phases you must idle, not doing anything.
2e figures the analog, Initiative with a simple d6. +1 is added for "cool guys" (airborne, ranger, SF, etc.) and the roll is halved for reservists (round up) and conscripts (round down). High is good as that is the number f phases you can act in. Combat experience is "limited" to the four years of the Twilight War you have experienced, even if your military career extends far enough you might be a veteran of Grenada or Panama or Iraq or the Falklands or Afghanistan or.... (Or even Viet Nam, in one deliberate example.)
But CUF in this version isn't these things. It is the ability to keep your wits when the world is blowing up around you. As, like 2e, you are a veteran of three years of high intensity fighting in the Twilight War, there is an assumption that all PCs should have reasonable CUF, otherwise they would be dead by now. So how do we figure it decoupled from rank?
The salient points are how much fighting the character has experienced. And that's really luck of the draw. I'm not opposed to a d6 roll as a final character creation step, with a kicker for SOF. To normalize the roll, I'd simply propose 1: D, 2-3: C, 4-5: B, and 6: A, with SOF (and maybe assassins and paramilitary) adding 1 to the roll.