Nape Resulting Impulses

Question

I'm trying to make the player body, when certain condition is met, to go up few "meters" then stop slowly in the "air" and come down.

The gravity is 600

public var gravity:Number = 600;
public var space:Space = new Space(new Vec2(0, gravity));

I thought that for it to go up, the impulse.y should be at least -601, to contradict the force that pushes it down. Although, setting -600 (or even smaller) will shoot it upwards never to be seen again. (even -100).

Yes the impulse is in the update function which means it will constantly add the impulse making it faster per tick. However, when setting -20 it doesn't go up in the y, (it should eventually as impulse is being given to the body per tick).

switch (combo)
{
    case "W":
    break;

    case "WW":
    impulse.x = 0.0;
    impulse.y = -powerJump;
    mainChar.applyImpulse(impulse); 
    powerJump -= 5;
    break;  
}

This is not finished and it is not what is described above but it's the gist of it. When condition is met the player should go upwards and the impulse should lose it's force. The problem is I don't know how much is sufficient to counter the gravity impulse. I can do it by trial'n error but I'd rather know how it works, how much impulse is equivalent to gravity=600?