You will get lots of different answers on the tri tip. These suggestions are only mine, but I have done many of them over the years.
Tri tip, coming from the bottom sirloin butt is a great cut, but not the most tender on the animal. I like med rare to rare meat, but I find the tri tip does a little better at med rare a little toward the med side. About 140 to145 before resting. No need to foil these, you can just put a loose tent of foil over the top and let them rest about 15 min. You should have meat running from med rare in the middle to med well at the ends, so differing tastes will have their choice.
Left overs make great sandwiches either cold or gently rewarmed in au jus.
Enjoy!