Startups and Angel Investment

I'm starting a startup with clear methods of generating revenue and although I hope it becomes successful beyond my wildest expectations, I honestly don't have any clue as to how successful it will become and how many customers I will be able to get and how long it will take to get them. I don't think any startup can know these numbers.

I'm speaking with investors about receiving some angel funding. Without an accurate projection of revenue and expenses, how do I know how much of the company to offer them?