The stirrup under the arm pit is an ok way. What I like to do is sit in the saddle, put my knees in the knee roll where I want them and stretch through my calves and get my heels down and under myself. Then just set your stirrups so that you are sitting the same as that :) When doing flat work you want as much calf on the side of your horse as possible so make your stirrups as long as possible.
I was taught the stirrup in the armpit thing too but it still comes up a bit short that way. Sometimes I'll start like that, but once I get on I will look at where my foot is in relation to the stirrup while it's out of the stirrup... should be able even with my ankle I was told
My understanding is that the Australian stock saddles have a more Western style stirrup... in which case it's more or less long enough to accomodate the ball of your foot and have your heel end up either parallel with the ground or slightly downward.
I personally use english stirrups on my aussie in a dressage type length... but I don't know whether that's adviseable.
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!