Hey guys I am going to go take pictures of waterskiing, slalom, barefooting, trick skiing, all that stuff. I will be doing panning shots with the slalom, the barefooting is just mostly behind the boat so not really any panning there. I will be using a 70-300mm sigma lens. I dont know what focal length i'll be using just yet, I do know the skier will be from 40 to 75 feet away from the boat. My question is what shutter speed do you think I should be using?