I'm running my mtb's with 34-24 chainrings and the FD's are more than 25mm above the middle ring. It is basically set where it would be if I had an outer ring.
It is a bit of a catch 22. It shifts fine as the inner derailleur cage is aligned at the correct place to move the chain from inner to middle; the FD doesn't know there isn't an outer ring. I did install a bash guard in the outer position as a precaution against throwing the chain off the middle ring. That is the catch 22 part. I'm not convinced I wouldn't drop a chain over rough terrain with the gap between ring and FD cage so great.
I would guess a 36 to 38 outer would require a wide gap for the middle to outer. Chainrings with ramps and pins would help, but I would think the inner cage needs to be set correctly for upshifting from the the inner and middle chainrings.
John