I just had a quick look on the Apps store and put in Weight Watchers in the search, there's quite a few Points Calculators on there, better to looks through them to see which would suit you best. I use Weight Bot to track my weight, because it's the only tracker that lets you use stones and lbs. It gives you a graph tp track your losses and also gives you a projected time to target based on your losses so far. It's £1.19, so more than the other apps but I find it really useful!
Hope that helps, I'm not doing WW but love my iphone.