how to choice nozzle expansion ratio in upper stage?

[iwanttogohome]

I want to deisgn the nozzle in upper stage.

but, I don't know ambient pressure in upper stage.  --> In upper stage, ambient pressure is very low. so This is impossible or hard upper stage engine high altitude test and CFD simulation.

When the upper stage engine design, how to choice the ambient pressure in working altitude?

[DDE]

You can easily survive a degree of overexpansion, so your primary consideration would be physical feasibility. Your engine bell has to fit into the vehicle's interstage, and you must be able to actually build one - all without performance gains getting eclipsed by extra mass.

[cubinator]

Yeah, there will be a balance between added mass and expansion ratio gains. 

Probably you can calculate both wrt nozzle length and find where they intersect for maximum efficiency.

[mikegarrison]

I believe the actual question was what ambient pressure you should assume. I'm pretty sure mostly engineers just assume vacuum. The slight amount of pressure actually present at the altitudes where second stages are used is so close to vacuum that it makes no practical difference.