I know you bbq purists are disgusted that I’m doing this over a gas grill but I’m working with what I have and need some advice. What’s a good rule of thumb for knowing when it’s done without a meat thermometer?

It’s 2.6 lbs.

When I have done the Costco pre-seasoned tri-tip at home, I cook it on low heat for about 20 minutes on each side until the internal temperature reaches just over 130°