You should look at the total market for the parent product, and come up with a reasonable percentage of those users who will buy your add-on. For example, if t

You should look at the total market for the parent product, and come up with a reasonable percentage of those users who will buy your add-on. For example, if there are 5 million iPhone users, and 5% buy a screen protector, then you're looking at a total available market of 250,000. From here, you'll have to figure out what percentage of the market you can reach, etc.