For a 3 pound tri-tip, which if fairly normal, at 225F, I would allow an hour. It will get to around 125F internal, at that time, pull and sear.
Hmmm, I would add, I actually do not cook by IT or time, I do cook by feel. But, experience tells me that if you are at 225F, an hour for the main cook and reverse sear is an excellent way to go. There are many ways.
If you are going to do what most folks call Santa Maria style, then you would be direct heat, over a fire around 375F to 400F, you will want to place and flip the meat every 5 to 7 minutes, on the three sides, I know this is not what you have read, but, it will make sense. It builds the char on the surface like a direct heat method is best at, but, by frequent turning, the meat will heat slowly and evenly, pull at 130F, let it rest for 15 minutes. You are looking at a total cook time of 40 minutes or so.
If you smoke first then sear, you will be looking at 225F for an hour, as stated above, then rest and bring grill to temperature, if you have two grills going, this is easier, although a 5 minute rest between smoke and sear is good. Sear once the meat reaches 125F internal and sear over a very hot grill. No more than 3 to 5 minutes per side.
Searing first, then placing over low coals is another way to go, sear and then place into the cooker at 250F or so, run internal to 135F and pull, rest for slicing for 15 minutes. Total cook time will be around 35 to 40 minutes.
For a first cook, serve some apps and a little beer, if you run late, nobody will care