if Bat and Rat were both chosen, and you heard "At", you can't tell which it was

Only problem is that you’re probably saying something like “Bryan, b as in bat, r as in rat” and then even if you put it further along in the word, “ b as in bat, r as in rat” is probably still going to be pronounced distinctly enough to catch that it’s “br” and not “rb”. Something like “rote” would probably be a better word with r since it could easily get confused for “wrote” even with the r sound.