Badbru's the man! I did this as well, and as you'll notice, the numbers are about as consistent as a.......... as a thing that's not very consistent. Me, I wanted multiple Gt yields, since I could tell these missiles weren't going to cut it against anything with decent armor; they're only good for hunting down little ships.
But then, I also realized that most missiles are hard to spot, especially if they've been designed right, and it might not be a ridiculous porposition for one to get in close and detonate on contact, destroying just about any ship.