I like looking for alternatives along these lines, but I think “just when” is too easy to interpret as only the “only if” part. “just when” and “right when” as set phrases also tickle connotations around the temporal meaning of “when” that are distracting. “exactly when” (or “exactly if”, for that matter) might fix all that but adds two extra syllables, which is really unsatisfying in context, though it’s at least more compact grammatically than the branching structure of “if and only if”…
I like looking for alternatives along these lines, but I think “just when” is too easy to interpret as only the “only if” part. “just when” and “right when” as set phrases also tickle connotations around the temporal meaning of “when” that are distracting. “exactly when” (or “exactly if”, for that matter) might fix all that but adds two extra syllables, which is really unsatisfying in context, though it’s at least more compact grammatically than the branching structure of “if and only if”…