FCTM says: At typical landing weights, actual thrust required for a normal go-around is usually considerably less than maximum go-around thrust.
Boeing don't define "considerably less." Is that for instance, 10% N1 less than full GA thrust? Or, maybe 5%N1 less? Seems awfully vague which is possibly what the OP was getting at?
So what do you tell the PNF? How about: "Going Around - Set 10%N1 less than max GA thrust, please old chap and while you're fiddling with the thrust levers, you may as well give me Flap 15."