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I don't think 1/640 is too slow if that can buy you a bit less noise. I shoot NBA at that shutter speed (at ISO 4000), and while occasionally there is some motion blur, for the most part it gets the job done. I'd suggest sitting about where the three-point line meets the baseline. This gives you nice, tight access to action directly under the basket, a nice angle on the key/foul line, and pretty much allows you to have a good shot at anything within that half of the court. There's no doubt you'd have a bit more leeway shooting at f/2.8, but I'm going to assume your lens is the f/4 version, so you're doing what you can.