I looked again at the datasheet and found the formula. First time I had just done a search for IREF not expecting the datasheet to be calling it REXT. More haste less speed.
That bugged me, too. I had to look twice to see if there was ANOTHER pin called REXT. Don't know why they call the pin IREF and the resistor REXT without anything to tie them together. And on the datasheet I found the reference to the voltage coming out of the IREF pin being 1.25V was on the next page and something I had to read a few times.