High altitude adjustment for a small rib roast needed

No adjustments are needed. The main reason that high-altitude cooking is hard is the reduced boiling point of water at low altitudes. But here, where the meat will be under the boiling point of water for the whole cook time, this is hardly an issue. Check out this article from the Spruce Eats:

Dry-heat cooking techniques like roasting or grilling are not affected in the same way because high altitudes don't alter the way air is heated.... Note that the temperature isn't affected, just the moisture content of the food. So a grilled steak might be drier at high altitude than at sea level — even if it's not overcooked temperature-wise.

I don't think that the issue of dryness is a big deal here; since you are cooking a large cut, only the surface will dry out. In fact, increased surface dryness would be a benefit if you plan to sear the meat after cooking. I highly recommend searing the meat if you plan to serving it hot; browning adds great flavor and texture that I think you will miss otherwise.