When you approach the low end of the low-and-slow cooking methods, a concern that is normally not an issue in faster cooking methods starts becoming important: how long will it take the food to get to safe temperature?
You want the food to be above 140 F (60 C) for the entirety of the cooking process, for safety, although having that happen instantly is not possible. Therefore, you want the temperature to get to that threshold as quickly as reasonably possible, especially for the surface of whole cuts of meats where the greatest risk of pathogens are.
Therefore, I would urge you to err on the side of caution and safety, and use the extra coal at the beginning of cooking until the food reaches at least 140 F.
Once it gets out of the danger zone, and into the safe zone, you have much more freedom. However, barbecue is far from exact. I would not worry about a 10 or even 15 F disparity. Your final outcome will likely be indistinguishable.
Still, once the food is safe, if you wish to allow the temperature to be somewhat lower, that is the time to do so.