Your startup is worth 100.  So what is 51% of it worth?
jacquesm: Well, 49% really would be worth 49, but to hand over control that is quite a different kettle of fish. It very much depends on how much you trust the party you are dealing with, to me if you sell 51% you might as well sell it whole in most cases, so I figure 51% = 100 - (whatever risk you think is acceptable).Not sure if that makes sense like that, but that's how I would do the calculation. In other words, the risk factor determines to a large amount how close to the 'real' value of the 51% we'd end, if I think the risk is large (that the party that will now control the start-up will crash it or disappear into the night) then I want it all now.