How do i calculate a startup valuation?

Answer 1

It is very hard. If you are buying in, require a discount of 30-40%. That should cover both failure risk and a reasonable return on equity. You need to treat it like any stock valuation, read "Security Analysis," by Benjamin Graham or the more recent update by Cottle.

Realize that the presenter is being optimistic. You need to kick the tires and shake out the presentation. You need to test how reasonable the assumptions are and presume it will go badly.

If you are preparing one, get an accountant. It is too difficult to do without basic knowledge.