IMO, it's 100% pure speculation at this point. If you already had a committment from a distributor you might be able to make a better guesstimate. There also tends to be a general bias from distributors against video originated films - whether justified or not. Scoring a distributor is probably the single most important and often overlooked things in a film project from a business standpoint. There are things you can do to improve your odds in obtaining one but, unfortunately, no magic, sure-fire techniques. If there was, we'd all be rich. So, from the standpoint of a business plan all you can really do explain how you plan to sell you film (go about getting a distributor) and (if successful) what you think you can get for it. You can use other similar films as examples. Getting useful data on other films takes time, research, and money.
Check out a book like "Filmmakers & Financing" by Louise Levison. I think it and other books like it will point you in the right direction.