Not tested but pretty sure it would work.
//Assume you have leader 'plane' and you want to setPos 'wingman'.
_dist = 150; //Distance from leader plane
_offdir = 90; //Relative direction from leader. (90+ here)
//Find compass direction to spawn from leader plane
_compassDir = ((getDir plane)+_offdir) mod 360;
//Find absolute coordinates by adding relative to leader plane
_newX = (getPos plane select 0) + (sin _compassDir * _dist);
_newY = (getPos plane select 1) + (cos _compassDir * _dist);
_newPos = [_newX, _newY, (getPos plane select 2)];
//Move wingman plane.
wingman setPos _newPos;
Don't forget to give the plane velocity so it don't drop to the ground.
If you want to know the math more in depth then of interest is: "unit circle", "sin" and "cos", in general basic "trigonometry".