That sounds like an excessive gap to me. Might be flyable, but it sounds ugly. I would rehinge the rudder if you already glued it in. If you have not glued her in yet, just drill yourself some new holes to close the gap. If you have that large a gap, you have space to drill new holes on either the fin or rudder, and not intrude on the existing ones. My gap is only about 1/16", between the top of fin and the rudder. Jon