There are different approaches you can use:
Specifying paraxial ray angles:
In image space, the combination of the chief and axial ray angles determine where and how large the Exit Pupil is. The Exit Pupil is positioned where the chief ray crosses the optical axis (zero ray height) in image space. The size of the Exit Pupil is determined by the axial ray height at that position. You can specify the angles of these rays (paraxial) as operands in the error function.
Finding and specifying size and position:
The Exit Pupil size and position can be determined using the PXS command from the command line, by clicking on the text window icon, or by inclusion in a CCL. You can create user-defined operands that determine the position and size of the Exit Pupil and then force these values to desired quantities. Directions for creating user-defined operands are found in the online help:
Contents>>Optimization>>User-Defined Error Functions>>Operands>>SCP Components
and
Contents>>Programming>>User-Defined Support Routines>>User-Defined Operands