I wish I would have seen this earlier. Congratulations on great first prime rib cook. Even though I do it all the time, I stress out too when I cook for other people so I totally understand.
The great thing about prime rib is it's so forgiving. The temp of the grill really doesn't matter a whole lot. Whether the grill is hotter or cooler, each has it's advantages and disadvantages. As long as you hit your internal temps, it's GOING to be GOOD!