First Finger - Idle detent
Middle Finger - Starter
Left hand - Throttle
Is exactly how I was taught.
I can't see the logic in using your left thumb for both the detent button AND use the same hand to twist the throttle, as some have suggested. An incredibly awkward and disjointed way of going about it. In my opinion, you need to have full control of the throttle, which by logic, you don't if you're trying to maintain your thumb in one position while moving the rest of your hand about. Seems like a recipe to slip off of something inadvertently to me.
I always felt that with two fingers on the buttons, you're in a safe place with that component. Your main concentration should be on the turning of the throttle as you're more likely to #%*! this part up!
So long as you keep motoring that starter throughout until you get to that magic 58% whilst keeping a check on the TOT with the throttle (CECO & Bendix appropriately) there's no problem.
Whatever works best physically for each Pilot I guess.