The two black lines are the same length (produced with copy and paste). You can see that the visible portion of his middle finger would--if unfolded--almost extend as long as the index finger shown. Almost? Yes, the bone between the the first and second knuckle is pointed directly at the photographer. If the length of that bone is added, the folded middle finger would in fact be longer than the extended index finger.
Now show us his other hand, so we can determine just how relaxed JFK is in this picture?