Not as dumb as I thought, apparently...
or maybe dumber than I could've imagined!
Right now, pedal tones seem easiest with 13. Then I talked with a brass gentleman who told me (upon asking about the fingering of a pedal C) something along the lines that it is the same as Bb and is fingered open. And yet, I can't hit the note on the right pitch open (only with other fingerings, 13 being the easiest).
???