Tri-Tip supposedly originated in Santa Maria, Ca. Traditional prep calls for a rub with: 2 TBS granulated garlic, 1TBS sal & 1 tsp pepper. Should be grilled to med-rare. You can sear then go indirect or reverse but, if smoking do that first with IT reaching 120-123º, then let it rest off heat for 5-7 min before placing on a hot grill to sear. FYI, when on the grill watch for flare ups due to fat content.
Flat Iron: we use salt, pepper and garlic. Let it rest on counter for at least 45 minutes then grill. 3 min both sides on very hot sear grates then to finish out the markings on only the serving side, 2-3 mins on non-searing lower heat grate. Usually we lightly baste each side with a small amount of italian dressing after each of the first 2 turns. Total time approx 8-9 min usually delivers a perfect med-rare but it depends on size. Smaller and thinner ones need LESS TIME. They are very easy to overcook. When I got one of those I then to skip or reduce the last 2-3 min flip. The flavor starts to dwain beyond med-rare and it becomes tougher to chew. Let it sit at least 10 min before slicing.