I pretty much use a 2 finger power grip for most forehand shots, especially drives. Forehand is not my strong suit by any means, but I do find it useful when there's no room for a backhand pull or I need to get right.
So for me, it's both fingers (index and middle) pressing against the inside of the rim not flat against the underside of the flightplate. There's a few reasons I do it this way.... it minimizes the drag of the rim across my middle finger where calluses can form from flicking a lot, I can get a lot more snap by keeping my elbow close and pushing the rim with my fingers instead of letting the disc kinda slide out and for me, it helps keep wrist roll in check. It's hard for me to keep a good flat angle of release trying to chop at it with my fingers facing up the whole time. If you power grip it, you really just can keep your arm loose and flick it with your wrist and off the tips of your fingers, no run up necessary.
It's easy to put too much on mids doing it this way, and force them all the way over..... even overstable mids b/c they're just not meant to fly that fast. Get a stack of at least 4 or 5 of the same disc (I like Rocs) and just get to a field and test out what works for you. It's easier to feel and fix problems if you get a few of the same disc and flick 'em, concentrating on release angle. Most mids will need at least some degree of hyzer angle and you'll find that you can release faster discs much flatter and they'll take off out of your hand like a rocket!! (ideally)
Good luck and just have fun practicing it.