Smoking with a Weber kettle, I find that my temps are too high (near 300 or more) because I have to keep both vents completely closed in order to keep them lower (250) and I'm paranoid about keeping them closed for too long and snuffing the coals out.

Advice? How long can both vents be closed before it becomes a problem? Or should I not even worry about it? I'm using a Thermoworks DOT with a grill clip to monitor temps. First attempt was with a 2-2-1 snake method for brisket and this time (running right now) is just a basket of charcoal with wood chunks on top for spare ribs.