old shooter, I was taught thumb & forefinger, but use what gets you time & accuracy. 

My problem was with a cock on opening rifle, I couldn't get reliable torque to open every time.  Especially the year they issued really hot 30.06 Greek ammo.  I broke an extractor off my M1, and could hardly operate my 03 bolt with the butt in shoulder.

I was taught to push the rifle to the right and push the bolt up & left simultaneously, getting the strength of two arms involved.

When I get my 1917 running, it will ne interesting to see how a cock on closing bolt will work for me.