You should be fine.

Make sure the light is far enough away from the plants so they don't burn, lettuce doesn't like heat at all.

Lettuce is a shade tolerant veggy and the footcandles of light your 250 provides when 2-3 feet above the plants is about the equivalent of the outdoors on an overcast spring/fall day which is when lettuce does it's best. When you consider that many grow lettuce in window covered cold frames during the low sun intensity period of winter it becomes clear that lettuce will do just fine indoors, under lights.

Heck, nearly every hydroponic grower starts with lettuce because it is about the easiest plant to grow this way under lights.