I use two strips for lettuce, but they should be fairly close to the plants. Less than 12 inches away.

I also have a light meter app on my phone that lets me measure light intensity and plug in my light schedule to figure how much the plants are getting. Two strips seems to be plenty for lettuce when you leave them on 24 hours or close to it.

I posted some lettuce I grew last summer, but they were grown under this 100w light. That light was on 50% power and on a 12 hour light cycle, so that tracks with the 40w you'd get with the light strips on all day.