Jeff, maybe I misunderstood your post, but adding marinade to your tri-tip won't help with doneness - and usually, neither will time vs. weight; I've had great success with tri-tips using a Santa Maria-style rub, wrapping it overnight in plastic wrap, and grilling over direct heat on my OTG while using white oak for smoke; once I've achieved a good char, I move it over to indirect, and baste with a red wine vinegar/EVOO and spices mop, turning every 5 minutes... I ALWAYS use a thermometer to monitor internal temps, and, take it off at 145 F internal temp, wrap in double-layers of foil, and let 'er rest in a cooler - 155 F is how I like it, and the resting time will bring it up about 10 degrees after 30 minutes or so...
hope that helps -
Regards,
Rooster