Hi Kevin,
Generally I’d slide the cover to reveal the flame broiler towards the end of the cooking process.

Usually opened only for 30-60 seconds depending on what I’m cooking. It’s very easy to overcook the meat, or to burn it. The meat temp usually rises a bit quicker so you have to compensate for it.

Eg, for a lamb rack, you want an internal temp of 135F. So I’d slide the cover when the meat is around 120F, and monitor the temperature until it hits just below 130F. Moving the meat to get a nice char on each side. Then remove and let it rest.

The meat temp will continue to rise while resting. Should hit 135F when covered.