For me it tends to go with the amount of bite available. On the smallest, low bite tracks -2.25 is my starting point. However, tracks don't have to make much more bite or be much bigger for me to head towards -2.75 or more.
I tend not to change castor often once I've gotten something working. For a Triton I'd take whatever has been working on the kart (be it 7 and 10 or 10 and 13). If I'm on 10 and 13 and I find I need more turning power I might drop it to 7 and 10 but even then I'd probably only do that after I'd made several other adjustments.