If you are possibly rebuilding their product because it was developed in india and neither founder has any technical knowledge i would ask for 33%. You would be their technical founder and since it is a technical product they need you. As to why a higher amount is a fair deal, they need you because they have no technical knowledge and sales experience or savvy is no good if there isn't a good product to sell. Sorry for the quick response.
Thanks for the response. I don't feel I would deserve an equal share as they are also willing to use the resources of their company (staff) for selling the product.
Researching more on HN, I feel 25% is reasonable vesting over 4 years. That would mean after 4 years, their contributing would be 50% more than mine (37.5% each). I believe they can help bring sales which would make that reasonable.
