I always try to stop or pause when the calf's chest is coming out. Make the cow do the pushing then. It will clear the lungs out much better. You will see a lot of "stuff" come out the calfs mouth - sometimes. Make sure the bag is not over the face/head at this time. I think everyone probably breaks the water (if it is not done already) pretty much right away and gets the calfs face clear. We always did it inside the "calving barn" which had four 12' gates on hinges in that area. We would get the cow in the middle and let her lie down. Then tie off on the calf and pull a little to get a feel for how hard you have to pull. Then open the gate closest to the cows rear a little and tie off onto the gate, in the middle. Use the gate as leverage against the cow. Just push the gate open when the cow pushes. The only problem is if the cow gets up, you need to get the rope disconnected from the gate in a hurry so she can walk around. I always tied that as a half hitch.
Good luck.
