Trim the large chunks of fat (or have your butcher do this) from the tri-tip, and cut the thin silvery skin off if you can. Rub the salt all over the tri-tip.
Combine the remaining seasonings (except the olive oil) in a small bowl and rub into the tri-tip. (*It will seem like a lot, but that's a good thing!).
Pour 1/4 cup of olive oil in a large resealable plastic bag. Place the spice rubbed tri-tip in the bag with the olive oil and rub it in to coat completely. Squeeze out as much air as possible and seal. Place in the refrigerator for 5-6 hours, or up to 24 hours.
When ready to grill, preheat an outdoor gas grill to 350-400 degrees F. Remove the tri tip from the refrigerator and let it come to room temperature while the grill is heating.
Place the tri-tip directly over the flames on the hot grill. Close the lid and watch for flare-ups.
Cook approximately 8-10 minutes per side for medium rare. It can take up to 20-25 minutes to cook a 2 pound tri-tip, so check the temperature (120 degrees for rare, 130 for medium rare and 140 for medium), or in a pinch use the flesh test (see notes).
Place the cooked roast on a cutting board to rest, covered loosely with foil for about 10 minutes.
Uncover and cut in half, where the grain changes direction, then cut each piece against the grain into 1/2 inch slices.
If you don't have a meat thermometer, try the flesh test. It's not quite as reliable as a thermometer, but it will do in a pinch.
Different parts of your face will match the feel of the center of the meat at certain levels of doneness.
Touch your cheek with your index finger. This is what "rare" feels like. Touch your chin; this is what a medium-rare to medium piece of meat will feel like. Touch your forehead - this is about the way a well-done steak will feel.
I've been using this method for years and it's almost foolproof!