I'm not totally sure that you have to know the correct class in order to file a patent. I was under the assumption that it was the patent examiners job to ultimately determine this. With that said, I'm sure it's quite important to know what class you'll end up in and this should help you complete a patent search of other competitors in the same class. Also, if you have a budget for IP, a thorough patent search done by a reputable IP firm will bring back information on any potentially overlapping patents and you can check and see which class these patents are in and work backwards from there. Also, I'm pretty sure your product could belong to more than one class.