Clear terms preserve goodwill better than hopeful assumptions.

Hoping the other person understood it the way you did is not a plan. Goodwill is real and it is not a substitute for specifics, because when the situation arrives, the goodwill will be under strain and the details will be all that remains. Clear terms are what allow people to stay friendly, since nothing has to be adjudicated. Vagueness costs the relationship, not just the outcome.

When something feels too awkward to specify, that is usually the item most in need of specifying. Say it in one plain sentence and move on. The awkwardness lasts a moment; the ambiguity lasts as long as the arrangement.