140 as kota suggested will be pretty much medium by the way I measure beef. Personally I stop at 120-125, then tent with foil and rest for 10 minutes. The internal temp will boost to 125-130 during that time which is to me medium-rare. Don't stop below that. Rare tri tip doesn't work well.

Swamp has the idea. Make sure you have an instant read thermometer or better yet an constant read probe. Also, I'd add some garlic and onion powder to the salt. Smokers enhance the flavor of both without burning like grills can.