What about an edge that improves detect charm. With the edge you would know if your servant is unhappy or preparing to depart.
One of the more annoying things for me when playing the class was the guess work in those areas.

example

Adept of Charms

l angel

Your binding appears frail. -ie leaving within 2-3 hours

Through you binding, you sense the demon's growing displeasure. -This could just be a pecho.

if it takes more then a second or two for them to follow they could be happier. If you have to sit and really wait for them to catch up it is time to start making a circle.