Sounds like you found one of these “rare” internet people who give real internet advice. :D

Hmm, this is difficult to say. It depends a lot on what you’re doing. Let me make an example to clarify. And yes it is mostly guesstimation. You can make good estimations or bad ones. Depending on the data you have.

If you’re starting on a new product and you know how many people need to buy your product to be profitable. And you can show a list of people who say they would buy that product from you (significant size of the amount you need to be profitable). And you can show that the market for this is big enough to sustain your operations (e.g. there are significantly more people like those that already signed up). Then you can guess that your product might be successful.

That being said, starting a new business is always a risk. This is not to discourage you. Please continue. This is to give you a reality check. :)

Does this somehow answer your questions? Would need more information to give more concrete input.