After losing nine months to a legacy ERP migration, Santi Nestares went after the hardest part of the problem first and built DualEntry from the ground up. He tells Lightspeed’s Claire Zau and Sam Eisler why most startups attack an adjacent wedge and end up dependent on the system they meant to replace, and how a negotiation that started in a car ended with him arguing numbers on the phone in front of a TSA agent.