It depends on the question you are asking! You will need to estimate the conversion rate based on what you see in real life, when you go from alpha to closed b

It depends on the question you are asking! You will need to estimate the conversion rate based on what you see in real life, when you go from alpha to closed beta, how many people who said they'd take, actually take it. Formal LoI's are good if you can get them, but don't really mean much, since they are by definition no commitment. And at this stage, they don't know what they're getting either.