Cooking time is a function of the pit's temperature and your desired internal temperature (IT).
Naturally, the hotter you cook it, the faster the IT will climb. At any given pit temperature it will reach 120°F IT before it reaches 130°F.
My estimate for your roast at 225°F is 7 hours to an IT of 120°F. I would say 6 hours if you cook it at 325°F.
Many cook books give a time projection based on the roast's weight. I find that their 22 to 24 minutes per pound is too long. And think about this—your roast weighs 19 lbs. At 24 minutes per pound it calculates to 7 hours and 36 minutes. If you roast half of it, 9½ lbs., then, by that same formula, it would cook in half the time. I don't think so.
To solve this time/temperature dilemma I always start my large smoke-roasted beef very early. If it's done early I wrap it in foil and a towel and put it in an insulated cooler (commonly called FTC). It'll stay hot and the IT will rise as the heat conducts from the exterior inward.
So, my final advice—
● Roast it at 225°F.
● Start 9 hours before you want to serve it.
● Pull it when the IT reaches 120°F.
● FTC it until it's time to carve and serve it.
You'll get contrary advice 'cause we all have our favorite ways of doing things and our pits and tastes vary. Whatever method you choose, please tell us what you did and how it turned out. Photographs are helpful.
EDIT: The carry-over heat will raise the IT almost 10°F when FTCing.