Immemen is not the only merge move and that's not the best move either. In fact, it can get you killed.
How aggressively you pull this vertical maneveur and whether or not you should depends upon the plane type mismatch.
I would recommend diving below the enemy first and establish vertical separation.
If they choose take the head-on, they will be diving which will cause them to pass through your altitude - increasing your E advantage. And it will cause them to increase their speed, which will increase their turn radius - increasing your angles advantage.
If they choose to zoom or immemen on the merge, guess what you are below them with speed, you zoom up on their six and take the shot.
I actually prefer the oblique 70 degree immenen as it adds angles as well as the the gravity assist coming over the top - and some times the other guys looses sight of you.