Cooking lower and slower allows the steak to pick up the smoky flavors coming from that nice grill you spent all that money on, and it provides a wider margin for error as the internal temperature will rise slower. It also makes for easy and exceptional browning because the outside will have more time shed moisture. Without that moisture, the outer edges will be free to exceed temperatures of 280°F—the necessary level for the Maillard (browning) reaction to occur. Adapted from Big Green Egg.
Dilute the barbecue sauce with 1/4 cup of water and rub all over the tri-tip on a baking sheet.
Pour any remaining sauce on top of the meat and allow to marinate in the refrigerator for at least 4 hours.
Remove the meat from the refrigerator, then set up a grill for indirect cooking and heat to 250°F. Cook the meat until it reaches an internal temperature of 115°F—cooking time will vary but it should take roughly 30-40 minutes. Remove from the heat and cover with foil while setting the grill up for direct heat and raising the temperature to 500°F.
Sear the tri-tip for 2-3 minutes on each side.
Let rest for 10 minutes before slicing and serving.