Not a stupid question at all
. I don't think headroom is going to be an issue in your grow...but there are other factors that can determine how long you veg for.
Basically, a 400 watt light will support a 3' X 3' wide by 12-18" deep box that you want to fill with flowers. Anything outside of this box will generally produce less desirable, smaller "popcorn" buds.
So you want to vegetate long enough to fill this box with buds. How long you vegetate will depend on how many plants you are growing. If you look at the diagram I attached you can see what I'm talking about.
For the typical bushy Indica that has a dense canopy with fat leaves your penetration with a 400 watt light is going to be low - maybe 12". So for a 3' plant, only the top 1' will have perfect lighting for buds; any vertical growth beyond that one foot for the flowers is more or less wasted, UNLESS you need to fill the horizontal canopy area.
When planning an Indica grow, I generally try to shoot for 2-3 main colas per square foot to fill the horizontal canopy area (the grid in my attachment). And how many plants you are growing determines how long you need to veg to fill that area.
For instance, if you were doing a SOG - lots of plants with one main cola each - you might grow up to 27 Indica plants in that 3'X3' (9 sq ft) area. It would require minimal veg time - hell many people go straight to 12/12 with a SOG - to fill your horizontal canopy area.
But if you were only growing one or two plants in the same area you would have to top and train - which requires more veg time - to fill that horizontal canopy area. That's the idea behind a ScrOG - using minimal plants in a screen to fill the horizontal area with bud sites.
Hope this helps..