Combat percentage during war/conflict?

While watching a documentary on Vietnam it was stated that while 555000 troops were in country in 1967 only 20% actually saw combat. I was wondering if this was the norm of those serving?

I don't have a source but have seen somewhere that every MOS in the Marine Corps is to support the 0311 and only 10 percent of the Marine Corps end strength is 0311, rifleman. That probably should include the entire 03xx field. Is the 10 percent accurate? I'm not sure but probably close. Is there a Marine MOS which does not exist to provide support for the Marine grunt? Maybe but I would like to see the evidence.

It depends on how one defines "combat." Are tanker crews leaving from Qatar to refuel over Iraq in combat? Are the F-15s and F-22s patrolling in combat? How about the security forces guarding the gates at bases in Afghanistan? Some answers are clear--SOF doing direct action raids, gunships and bombers dropping munitions, artillerymen firing on ISIS, etc. Some are less so.
Even then, percentages are going to vary by service and MOS/AFSC. The AF has many more people supporting than actively engaged in direct fighting than say, the Army...although even that depends on the style of combat going on at any given time.

Think of it as a spear ...the pointy end (ie. the tip of the spear) is what causes the damage, but it takes a whole lot of support and infrastructure to make that spear work. I would expect that 20% is about right now.