I agree with Bruce's recommendation, but would suggest more or less flash depending on the ambient light. That is, 1:1 or so in bright sunshine, but -1 or -1.5 stops in overcast conditions. That way, the flash will be just fill, not primary light source. Keep in mind that in very sunny conditions, the flash may only reach 10 or 15 feet (3-5m).
If you're shooting in the barn, the flash will probably be the primary light source, so the regular TTL setting should be correct. Hope this helps. Pat White
