Shaverg, I use my middle finger, and a special thimble from the Thimblelady. You can find her website if you google "thimblelady". I use her method for quilting, and if you try it you will never have problems with your under hand fingers hurting or getting callouses again. It is a very pain-free method of hand quilting, and I find it to be absolutely glorious!
Not only does my under finger not hurt anymore, my right wrist has stopped hurting from the stress and my hand quilting is much more even and small.
I can't really say enough good things about the method, it is wonderful. I even sprang for the sterling silver thimble she sells, after I tried the plastic ones to get the right size. She sends you a complete size selection so you can try them all out, and they were well worth it. I gave one to my daughter after I tried them, and now she is using the new method and loves it. I'm gonna get her a silver one for Christmas this year.