Have to agree with fezzi's post here. Try to find a way to talk to customers. You should be talking to them anyways to get their feedback on the idea behind your product.
If you must use beta users as a metric, be very conservative and realize that most of those signed up will probably not even use the beta, let alone buy the product (this varies by industry and product of course, among many other factors). In your case, you might think 1 in 100 signups will become customers, or 1 in 1000, or 1 in 10. Since we don't know what the product/market/marketing methods etc are, you're the best one to make the guess.
But yeah, you should talk to real potential customers.
If you must use beta users as a metric, be very conservative and realize that most of those signed up will probably not even use the beta, let alone buy the product (this varies by industry and product of course, among many other factors). In your case, you might think 1 in 100 signups will become customers, or 1 in 1000, or 1 in 10. Since we don't know what the product/market/marketing methods etc are, you're the best one to make the guess.
But yeah, you should talk to real potential customers.