Some formula I forgot, like add 8 unlit coals every 20 minutes. Or 40 minutes?
Getcha a cheap oven thermometer you can leave on the cook grate away from the coals. When it starts to fall, add a few unlit coals before it falls too much. Don't add more than 8-10 at a time or you'll overshoot the target, I think. Give 'em time to light before panicking.
Vents play a role here. Depending on how many coals are present at any given time, you many be running wide open with fewer coals, or partially shut down with a lot of lit charcoal in there. The latter approach is probably easier to regulate.