Thanks for the info. When I got home today, I went out to the back yard to try some casting practice, and I think I finally realized how little power is really needed for a simple short cast (around 30 feet of fly line). I remember reading that if you can hear a swooshing sound from your rod, you're using too much power. I worked on using as little power as necessary, and when I got it right, my backcast seemed to make itself with a nice tight loop!
I was probably only using 1/3 to 1/2 as much power as I had been using. Also, my casting stroke was much shorter, and the application of power slower. Doing this, the thumb on top grip seems comfortable as well since I'm not trying to overpower the cast.