I have only recently noticed the back fat giving way to the weight loss and I'm at the 25 pounds lost point. I dropped inches in my waist and bust first, then last week I noticed my back has smoothed out almost completely. Keep an eye on your bust measurements and I'd say when you start seeing those creep down, that may be the best judge of when your back will slim too.
Also, I do pullovers. I don't do mine on a medicine ball or anything fancy - I just stand upright and use one 5 pound dumb bell for resistance. You can find a few different beginner examples abound online.