Its really based on the feel your going for or your own STYLE I usually use between 18-20 for lines when going for a realistic style filming but TBH i've really been enjoying the 10mm wicked hoverboard type filming simply because its somethin different and makes it fun now that the ghosting's gone u can really play with the angles and make the distortion work for something more entertaining.......... The best thing you can do is find something you like and the feel you want and just go with it trust yourself i mean your vid is your vision right so play with em all and do what you wanna do