It all depends on the saddle and what stirrup leathers you're using. The arm pit thing works on most GP and J saddles, but as everyone said, D saddles you need to be longer. Some stirrup leathers have different hole lengths, or the leather's been stretched due to wear, so the length you were used to might no longer apply with this particular stirrup leather. I know that I've had to have two different lengths on the same pair, 5 years old, because one had stretched and the other hadn't!
Oh, and if it all goes pear-shaped, you can always fix it when you're in the saddle! I know I'm always fiddling with mine just after I get on!
The art of keeping a horse between YOU and the GROUND!