How much to move forward on each mouse wheel forward scroll. For instance, passing 20 here will decrease the distance to the XY plane by 20% each time. The zoom out amount will be set in such a way that it undoes the same effect. For instance if 20 was passed here, zoom out amount will be 25% since zooming in by 20% and then out by 25% of the result gets you back to where you were.