The angle between two planes is determined by the angle between the normals of the two planes.

https://www.cuemath.com/algebra/angle-between-two-planes/

https://byjus.com/maths/angle-between-two-planes/

I do not know which MoI commands would yield the surface normal of a plane.
Some clues here:
https://moi3d.com/forum/messages.php?webtag=MOI&msg=9581.321&highlight=yes

- Brian