I've cooked a lot of roasts on the BBQ over the years. I almost always use indirect heat (burner on the left, meat on the right) and I always monitor the internal temperature using a wireless thermometer. Here is my usual setup.
This was a 2.75 LB/1.25 kg top siloin cooked at 300F for 2 1/2 hrs. I've done this often enough and plotted enough graphs to be able to predict within 5 minutes when my roasts will be done the way I like them.
You can see in the graph how the internal temperature rises on a curve. The curve is always the same shape and the slope is dependent on the BBQ temperature and the size of the roast. (some people think I have too much time on my hands!)