Hi and welcome to SMF, I grill mine Santa Maria Style over a medium heat, 7-8 minutes per side until I reach an internal temperature of 137°, tent it with foil and let it rest 10-15 minutes to let the juices redistribute, we like ours medium rare to medium.
If your interested in Santa Maria Style what I do is sprinkle all sides of the meat with salt and let it sit until the salt dissolves and draws the moisture to the surface (this helps to hold on the spices), 10-15 minutes, then sprinkle on garlic powder (I cut the top off of a head of garlic and drizzle on some olive oil and then smoke 3-4 heads at a time, after it's smoked I dry it and grind it into powder so I have smoked garlic powder on hand) and ground black pepper, after that I pat on dried parsley and on to the grill it goes.
Enjoy your Tri-tip however you cook it and be sure to post some Q-view.
Gene