Projecting sales figures

[Clover]

I am setting up a web based business selling clothing and accessories to a niche market. Can anyone give me some advise on the best way to go about arriving at my sales figures for my first year to put in my cash flow forecast. This will be part of my business plan which I will need to get funding. Is it just a case of 'think of a number' (? !) or is there some sort of accepted forumla that I should use - for example a % of my perceived customer base. Would appreciate advise from anyone but particularly if you have had experience of this type of business and can let me know how your first year sales panned out. Thanks