Scrog screen before or after flower?

Hi. I was wondering with my four plants under my 400 watt hps, at what point do I add my screen? Do I top/fim, veg, flip lights then add the screen for them to fill in flower or do I veg into the screen then flower? The strain grows big plants and is 70% sativa so I am expecting some stretch so I would need to leave a good amount of screen space for them to fill in during flower. Am I on the right lines?

Thanks

[video=youtube;o6TdFKfIcac]http://www.youtube.com/watch?v=o6TdFKfIcac[/video]