CC David Smith re rolling quick ratio. I am glad it has been helpful. Re the second question of coming up with the window that works for the product/vertical: can you please be more specific as to what product you are working on so that I can give you the feedback?
Attribution is an issue esp for the services with a big number of non logged users and low frequency usage. You will have to start adding new tools to get resurrection right — Branch, Singular are doing a good job with user level attribution — as no off the shelf analytics tools will do it for you.