Why not try at 100%, this would accelerate growth and increase bud density when you do go to 12/12.
Stretching usually occurs during the first 2-3 weeks of flowering, not during veg.
Change to flower when you think the plants have filled your grow space adequately and the canopy of your plants is at about 40-50% of the maximum height that you have in your grow set up.
Hope this helps, Organoman.